To echo Ace, the remnants of the old Roman Empire would be using late Roman weapons and tactics. Ace would know far better than me what that actually involved, but there is some very tenuous evidence to suggest that the British and their Gaulish neighbours of the period relied on fast moving, small cavalry units. From the 5th Century on, shield designs started to show an increasing Christian influence.
By contrast, the Germanic peoples relied almost entirely on infantry. The favoured weapon was the ash spear or the axe. My guess is that only the leaders and their personal retainers would have had much by way of armour, but if you google the Sutton Hoo helmet, you'll get a very good idea of what they might have looked like. Swords were also limited to the warrior caste.
From the Germanic perspective, there would not have been any uniforms. Local warlords would have been surrounded by their war band (as would their Celtic counterparts), but otherwise their forces would have been made up of relatively poorly equipped farmer-soldiers.
As far as Britain is concened, my guess is that the respective methods of warfare basically ensured stalemate in those early years until Badon. The British were never likely to drive the Germanic Saxons out entirely, as although relatively small numbers of highly trained and well-equipped cavalry can win battles against huge odds and cause great confusion and fear into the bargain, they cannot hold large swathes of hostile country. On the other hand, the Saxons could never consolidate their earlier gains for fear of what might happen to their farms and their families at the hands of the British cavalry if they were campaigning too far from home.
Regards
Peter