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I did a few of these races a couple of years ago. They are expensive but nice races in hilly places with a range of distances. ( http://www.endurancelife.com/event-new.asp?series=85 ).
Also: North Downs, South Downs, Ridgeway, Chilterns, Cotswolds are all within relatively easy driving distance of London.
Look at the XNRG website and maybe the Centurion Running websites - both of those organise ultras in those areas and they might give you some route inspiration, even if you don't fancy running the full distances.
http://www.xnrg.co.uk/events.aspx
http://www.centurionrunning.com
Treadmill climbs don't really compare.
I propose a number of weekend trips to places with hills.