In August 55 BC, Caesar invaded southern England, officially in order to teach the Britons a lesson (he argued that they had helped his enemies in France).

Things did not all go Caesar’s way, however, his heavily armoured Roman legionaries having to jump from their ships and wade through the sea towards a well-prepared, and drier, foe. Using ship-mounted artillery, Caesar was able to clear a part of the beach and, with Roman soldiers rallying around their standard bearer, a beachhead was finally established.

We don’t know exactly where all this happened, but it is traditionally thought to have occured somewhere near Walmer in Kent.

Archaeologically speaking, the earliest battle evidence from the British Isles comes from the Neolithic or New Stone Age, when the farmers were first competing for limited resources. At Crickley Hill in Gloucestershire and Hambledon Hill in Dorset, the earthen ramparts of two settlements were attacked, some time around 3300 BC, and their defences partially levelled.

Large numbers of flint arrowheads have been found at the entrances to both sites, probably where the fighting was fiercest. Unfortunately, in the absence of written accounts, we have no way of knowing who was fighting whom (nor why).

Source:

http://www.historyextra.com/article/what-earliest-evidence-battle-britain

http://97.105.52.213/en/artical/21231/What-earliest-evidence-battle-Britain