When you add up the total cost of US, may not be much cheaper than UK. You could also strike a deal with aclub here for 70 hrs and get a cheaper rate. You're much better off doing your hour building in the same environment as where you will do your CPL - I did and it makes a heck of a difference. You get to know landmarks, airspace, and the local wx etc. Alot of the CPL is nav routes and if you practice these in your hour building. May save you some hours on your CPL course.
Either way, make sure you bet a brief from your intended school on what to prcatice and make the most of your 70 hrs - don't just go boring holes in the sky.