Your blood pressure may be problem in the issuing of a Class 2 medical so it would be best to get that done before you embark on any training. You can find a local medical examiner
here
The length of time a medical certificate lasts depends on your age; the details will be on the cAA website.
The best thing to do would be to decide what you want to do with your licence once you've got it. If you plan to self fly hire afterwards, then learn on a type which is locally available. You have to learn on one type through the PPL course so you could learn on an R22 and later get an R44 rating. Or you could learn on a SChweizer or Enstrom if they are used by the school you wish to go to.
In order to keep your licence current you must fly two hours a year and take a licence proficiency test annually. The licence is valid for five years but all that means is that, providing you keep current, you pay more money to the CAA and get new pieces of paper. You don't have to redo the theory or full PPL test.
You need to have passed all the theory exams before you can take your PPL skill test and most schools will require that you pass Air Law and Navigation before you go solo. Best to talk to your school about how they operate their exams but there's no harm in getting the books now and starting studying.
The FAA is the USA equivalent of the CAA and JAA is the European equivalent. If you wish to fly in the UK by hiring a local school's machine, then you would probably be better off getting a JAA licence in the UK rather than going to the States and getting an FAA licence. There are many complex rules about flying UK registered aircraft in the UK under a foreign licence.
However, if you are looking to buy your own aircraft, then there may be more cost-effective ways of getting your licence.
Hope some of that helps; speak to your local flying school and google for many answers.
Cheers
Whirls