Medical requirements
You cannot fly an aircraft solo EVER without holding a CAA APPROVED medical. This may be a medical certificate issued in accordance with the JAR's for which a medical undertaken by an approved AME is required or alternatively a 'self declaration of health' countersigned by your GP which is the minimum requirement for the NPPL.
What some of you appear to be saying is first go the self declaration route signed by your GP which is cheapest and later for the licence issue should it be for EASA/JAA then have the AME medical. You should remember that effectively, in the end, this way, you will be paying for two medicals. Most GPs charge for signing your form and may require you to have a full medical with him/her anyway. OK, it is true, you don't have to declare for which licence your training until preparing for your QXC and later the final test.
I mention the QXC for this reason. For the EASA/JAA licence your instructor may send you on your Qualifying Cross Country (QXC) whenever your considered ready. You must for this hold a EASA/JAA medical. For the NPPL before you undertake the QXC you must first pass a Navigation Test (NST). With the NST passed you then only require a valid medical self declaration. Should you be sent on your QXC holding only a 'self declaration, but having not passed a formal NST with an approved examiner, you may be illegal. Additionally should you have followed the NPPL route for the QXC but elect later to go for the EASA/JAA licence you must do a second navigation test, part of the EASA/JAA testing requirement - your earlier NPPL navigation NST will not count. However this is my personal opinion and I await the alternative arguements, if there are any.