Going to the states at this stage will cost you more than finishing in the UK, and the hassle involved in visas etc means you won't be able to get over there quickly anyway.
I presume if you're working your nads off you're pretty tied up during the week for lessons. When I was learning and frustrated with the weather I would book 4 lessons every weekend. Early morning and late afternoon on both Sat and Sun. Most of the time you'd get at least 1hr a week, sometimes all 4. Also, if you're progressing well and need to get the hours in there's no reason why you can't do some flying at night and bundle a night qualification into your 45hrs. A lot of schools will recommend against this because it will deprive them of an extra 5hrs revenue, but if you're on schedule to be ready for test before your 45hrs are done it's worth considering as it's a way to fly during the week, even in winter.
Nav isn't too tough, it's mainly practical stuff working out routes and stuff. It's based on the southern chart, so as a shandy-drinking Londoner you hopefully won't have the problems I had (ie. finding the
airfields!). If you can use a whizz-wheel properly you're laughing. Technical requires a lot of knowledge, but there's handy assistance in the form of
this which you can think of as a Bristol QB for PPL.