The approximate number of boats  in the fleet that carried the invading Roman army to Britain in 54 BCE. Of these, 28 were dedicated warships and most of the rest were troop transports. They used for troop transports both Standard warships and merchant ships, likely more merchants (when it’s not mentioned) than warships, which would’ve been less effective and more unusual. But there are definitely plenty of examples of both.
In 55 and 54 BCE Julius Caesar mounted two expeditions against Britain. Although he met with only limited success and did not establish a permanent Roman presence on the British Isles, he did establish treaty relations with many British tribes and drew Britain into the orbit of Roman political ambitions.
An anti-Roman revolt by the Veneti of Armorica (in modern Brittany) in 56 BCE, which probably received some support from Britain, led Julius Caesar to turn his attention northward. To enter Britain, an island that lay impossibly far off, beyond “the bounds of ocean”, would have brought him immense prestige. However, political difficulties delayed his invasion plans for a year. Finally, in 55 BCE, Caesar prepared to cross the Channel with a small expeditionary force. His principal opponent was to be Cassivellaunus, who was probably king of the Catuvellauni, a tribe that had been expanding from its base at Wheathampstead in Hertfordshire to dominate much of southern England.
Caesar received envoys from a number of other British tribes who were eager to show their submission and thus avoid having their lands invaded. He also despatched a small reconnaissance force under the tribune (a senior military officer), Volusenus, to scout out suitable landing beaches. In addition, he sent out a diplomatic mission under the Gallic chieftain Commius to further rally pro-Roman opinion. Unfortunately, though, both failed— Volusenus was unable to locate a sheltered harbor for the Roman fleet and Commius was promptly arrested. On August 26, Caesar set sail with a force composed of two legions—the Seventh and Tenth. The cliffs and beaches around Dover were occupied by British defenders and the Roman ships were forced to run aground somewhere near Deal in Kent.
The legionaries had to disembark in relatively deep waters under a constant hail of missiles. Although the legions managed to establish a beachhead, disaster struck four days later when a severe storm scattered the ships that had been bringing more than 500 cavalry as reinforcements, and also badly damaged many of the landing craft. Deprived of cavalry support, Caesar was vulnerable, and after the Seventh Legion was severely mauled in an ambush, he chose to declare the expedition a success and returned to Gaul accompanied by a number of British hostages.
A new expedition
Preparations were soon underway for a new expedition. Caesar had learnt lessons from the comparative failure of his first incursion into Britain. This time he decided to bring five legions—amounting to more than 30,000 men—and some 2,000 cavalry. The latter were a critical component in countering the battle tactics of the Britons who, unlike their counterparts in mainland Europe, still used chariots in battle to harass infantry units, which lacked mounted support.
On July 6, 54 BCE, Caesar set off for Britain once more. His flotilla of 800 ships landed near Deal, this time unopposed, apparently because the Britons were so intimidated by the size of the force that they chose not to resist it. Once more, however, the Roman fleet was battered by a serious storm and the 10 days delay in building a rampart extensive enough to allow the remnants of their naval force to be beached, emboldened the Britons. They were then able to offer a more effective defense under the leadership of Cassivellaunus.
The Romans won a series of engagements. They captured a hillfort at Bigbury near Canterbury, overcame an attempt at entrapping a Roman foraging force, and then pushed on toward the Thames. Diplomatic pressure also began to tell now, as Caesar had with him one of Cassivellaunus’s arch-enemies, Mandubracius of the Trinovantes. Some British chieftains who were afraid that Cassivellaunus might use success against Caesar to increase his own power also began to waver in their support for the campaign against the Romans. The capture of Cassivellaunus’s chief stronghold—probably the oppidum at Wheathampstead—led to a desperate attempt to stir the Kentish tribes into a final uprising against Caesar. It was to no avail and Cassivellaunus sued for peace. Caesar readily accepted, as he had already decided not to overwinter in Britain, fearing that a revolt might break out in Gaul during his absence. He accepted British hostages and fixed a tribute to be paid by Cassivellaunus before returning across the Channel some time in the middle of September. The Trinovantes became, in effect, a client-kingdom of Rome. In addition, Cassivellaunus was forbidden to interfere in their territory.
Whatever his intentions regarding a third and more decisive invasion of Britain might have been, Julius Caesar was distracted from taking any action until 51 BCE by a major uprising in Gaul and thereafter by his involvement in the Roman civil wars, which led to his appointment as Dictator in Rome in 47 BCE.
[Extract]The Roman Invasion of Anglesey
By John Griffiths
‘All the machinery of war that had been used successfully throughout Europe was thus brought to the field of coming battle. Paullinus had also brought small flat-bottomed boats which he planned to use to support his advance into Anglesey. Unlike the previous assualt on the Isle of Wight, which had been aided by vessels of the Roman Navy, the situation concerning Anglesey was different. The Menai Straits is a narrow strip of water dividing Anglesey from mainland Britain – but it is tidal and fast flowing on both flood and ebb tides.. Under other circumstances, the Romans would have connected bridgeworks to these boats to enable to infantry and cavalry to land on the far shore. The spot chosen by Paullinus was some 250 yards across – but Paullinus had reasoned, correctly, that bridges would be useless. They would be wrecked in a very short time by the action of the swift flowing currents. His plan was to use the boats as landing craft for his infantry, crossing at slack water, supported by cavalry swum across – and all covered by a bombardment of fire and stones. With this in mind, and in full view of the Celts on the far shore, Paullinus made his camp at what is known today as Llanfairisgaer, outside of modern Caernarfon.
The boats were launched. Accompanied by the cavalry who swam their horses over, the legions began their assault on Anglesey. What a sight it must have been! The beetle like craft slowly making their way across the slate grey waters of the Menai Straits. Armed cavalrymen on horseback breasting foam as they swam over to meet the enemy. More cavalry swimming across between the many boats containing the heavily armed legionaries. The air filled with the whine and shrieks of missiles as they flew from one bank to the other, their dull impact raising both earth and the shrieks and moans of the enemy when they landed. The shouting of Centurions and Principales. The screams of the tribesmen and their supporters, the clash of metal swords on shield the mad martial music of warfare.
Yet, even before they had landed, there would have been, inevitably, casualties. Boats capsized, sending the heavily armed troops to the bottom of the Straits to drown. Arrows, fired in ranks against the invaders, would have struck horse and rider, would have landed in the small boats killing many and leaving the survivors to try and paddle for the shore – their work hindered by the deadweight of their fallen comrades. Perhaps the Celts threw their javelins as the first of the legion landed in an effort to stem the tide of invasion – but it would have been of little use. One can only summise that the first troops ashore would have waited until they were of sufficient number before starting to make their move. The coast there is low, gently sloping up from the beach, and the going – once the troops were clear of the water’s edge – would have been firm. The gradient is also gentle, not hilly, so any advantages the Celts may have thought they had would have been very quickly dispelled by the sheer number of troops coming ashore to form up for battle.’