There's the UK bike park down there i believe, not exactly XC from what I gather but having not been there I'm not sure.
Also look at vertebrate graphics SW Mountain biking for ideas.
http://www.purbeck.gov.uk/default.aspx?page=29&terms=cycle
The Isle of Purbeck is one of the most special places in the country.........I love it!
If you feel like walking for a day or two instead of biking, then take the coastal path at any point between Durdle door and Worth Matravers.....just stunning!