I'll second what a few people have said:
Roma Imperious (True 20 or Iridium System) from Hinterwelt
Eternal Rome (D20) from Green Ronin
Legends of Excalibur (D20 or True 20) from RPG Objects
GURPS Celtic Myth
And, of course, Pendragon, now in its 5th Edition.
Also, Coryani in the d20 Arcanis setting from Paradigm Concepts has a strong Roman fantasy feel and there are a couple of good adventures written for it, including The Bloody Sands of Sicaris, if you like pre-published adventures.
The d20 vignette book from Eden Studios 'Wonders Out Of Time' also has some good ideas for Roman-influenced scenarios, which could be used in a post-Roman British-type setting.
Post-Roman also, the d20 Seven Saxons adventure from Bad Axe games is high level but could also be integrated with a bit of work.