The FAA PPL oral is tough. That said, it does make sure you're a competent pilot. It also includes US regulations, some of which aren't relevant to the UK. But it does allow you to fly in the USA. I have a CAA PPL (pre-JAA) and an FAA one (with an IR attached). The writtens for the JAA are tougher than the US written, but the oral and the flying standards are harder for the FAA.
Once you get back to the UK, you can fly here on your FAA PPL - but you can't leave the UK (for example to go to France) without a JAR-recognised PPL, unless you're in an N-reg aircraft. There aren't many of those available for hire here these days.
I'd be surprised if you could do the FAA PPL for £4k now: I got back from two weeks of flying round California last month, and fuel was around $4 a US gallon in most places ($5.60 in Palm Springs!). The "surcharge" made it expensive to rent an aircraft.
You also need a visa, TSA clearance, and a flight school that's authorised to issue M1 paperwork. Many claim to be so, some of those aren't. Check carefully!