1. You need not wait untill you sell your shares. You could obtain a government career development loan and start training now. The £8000 will take you through PPL and your ATPL thoery, which takes most people about 9 - 12 months. The loan is interest free whist you train so if you pay it back when you sell your shares it will cost you nothing
2. With the weakness of the $ you can obtain an FAA PPL with a reputable US school for as little as £3250 including all of the extras. Note that you may need extra training to reach standard. Most people would spend no more than £900 on this.
3. Major advantage of the US is the cost and year round training in reliable weather conditions. It is not unusual for UK student pilots to wait for weeks when doing their solo cross country flights. Major advantage of the UK is that you can easily visit the schools before you commit. The quality of the training can vary substantially both in the US and the UK
If you are training for a JAA CPL it makes no difference whether you get an FAA or JAA PPL. Any ICAO PPL qualifies you to enter the JAA Theory/ME/IR/CPL modules. You will also be able to use your FAA PPL to hire UK aircraft in the UK as long as you do not fly it outside of the UK. When you obtain your JAA CPL it will include JAA private flying privilages, so the above restriction will no longer apply.
4. I will send you a private message recommending a school.
5. Your ATC training is more dependant on the school's airfield than on the country. If you train on an airfield which provides only a radio service, then the ATC training is likely to be poor. The US terminology is different to the UK so some familiarisation will be necessary on your return. Terminology however,is only about 10% of communication training. The main skills are about being clear and concise about your intentions and responding to the controllers efficiently. In the US you will have the opportunity (once you become competent) to fly into major airports amongst large commercial airliners. This is not practical in the UK.