Are we talking about the Invasion in 55BC and 54BC or the Invasion and Conquest in 43AD?
Although Julius Caesar wintered in Sequani (modern Lyon) many of his Gallic and Germanic campaigns were undertook in Summer and so it is likely he would have been elsewhere at the time. In 55BC, he sent a tribune, to scout the British coast in a single warship. He probably examined the Kent coast between Hythe and Sandwich, but was unable to land, since he "did not dare leave his ship and entrust himself to the barbarians", and after five days returned to give Caesar what intelligence he had managed to gather.
Following this Caesar gathered a large fleet at Portius Illius (Boulogne). Initially he tried to land at Dubris (Dover). However, when he came in sight of shore, the massed forces of the Britons gathered on the overlooking cliffs dissuaded him from landing, since the cliffs were "so close to the shore that javelins could be thrown down from" them onto anyone landing there. After waiting there at anchor "until the 9th hour" and convening a council of war, he ordered his subordinates to act on their own initiative and then sailed the fleet about seven miles along the coast to an open beach. In the absence of archaeological evidence at the landing point, this beach was most probably at Walmer. It was thought in the 19th century to be near Deal Castle - hence a house there named SPQR - but is now thought to be half a mile further south, where it is now marked by a concrete memorial.
In the latter, the Roman Army sailed from Boulogne in France across the English Channel and landed at Richborough in Kent. The British tribes met the Romans in a fiercely fought battle at the River Medway. After much bloodshed the Romans emerged victorious and 4 Legions went on to conquer all of England, much of Wales and parts of Scotland.
So generally speaking it could be 18 miles or 25 miles, or for a full invasion and conquest,