Assuming that you do the 15 hours in two weeks you are looking at 144 pound for riversides own accomodation (based on £11 per night) you can enter the us on the visa waiver as you'll not be undergoing any training (this is only hours building right?) this is merely a flying holiday. Personally i cant see why the school would let you solo the duchess IFR if you pass a check ride and have the neccasary licences?
Best people to talk to on this matter are riverside themselves on the number above or try this link. Remembering that a great majority of thei students are british
http://www.riversideflightcenter.com/
regards
d'vay