NPPL/PPL and all that
The term SSEA means Simple Single Engine Aeroplanes.
The Medical Declaration is a simple form on which you self-certifiy your health but your personal GP (or Doctor within the same Practice), with whom you are registered, is required to countersign it. The form may be downloaded from the NPPL website and has detailed guidance notes attached to inform your GP. However the standards are those of the DVLA Professional Drivers and will be well known by most GP's. The validy periods are included within the notes.
You may have a JAR compliant medical whenever you chose in the future.
There is no requirement to inform the CAA that you will be operating on a Medical Declaration. Your recorded status therefore remains unchanged, although should an official wish to check you will be recorded as a PPL with a lapsed medical. Should you be required to produce a medical cerificate, then you will, of course, be required to show your valid Medical Declaration.
If it all seems too simple then as BEagle saids, don't complain when the CAA choses to be so simply pragmatic. Well done the CAA.