EDIT -
The original site is now dead, it seems, but I found another superb resource on pre-Roman sites. IT's very comprehensive, and although the focus is on Megalithic sites across the UK, some other ancient sites are covered.
The interactive map takes a while to load - and maybe to get used to - but it erally is a great feature:
Modern Antiquarian - Map of Sites
The original site is now dead, it seems, but I found another superb resource on pre-Roman sites. IT's very comprehensive, and although the focus is on Megalithic sites across the UK, some other ancient sites are covered.
The interactive map takes a while to load - and maybe to get used to - but it erally is a great feature:
Modern Antiquarian - Map of Sites