I looked into hiring some girls a while back from a few modeling sites and actually contacted some. It seemed like the going rate for Artistic Nude was $50 to $100 an hour. For Erotic Nude or Explicit Nude it seemed to range from $100 to $250 an hour, depending on the model. I would say you could probably get away with offering $100 for a 2 hour shoot (for new or semi-pro models). Once you start getting into pro models you're looking at over $200 an hour just for swim suit shots. I was looking at American models in my area so the rate could vary for UK models.
AUS Models is a good site to browse and get an idea of some price ranges. Might even see one you like and one that has a decent price.
http://www.ausmodels.com/
Slick