I can't find the details right now (but if you check the NPPL website I'm sure you'll find something) but I'm fairly sure that you can fly on your PPL with a declaration as long as you stick to NPPL rules.
From the NPPL website:
The CAA have confirmed that JAR (expiring) and CAA (non-expiring) licences
(PPLs, BCPLs, R/RCPLs and any other pre JAR-FCL CAA-issued pilot licence) can be used with a NPPL medical to fly with privileges of the NPPL. However, it should be noted that the JAR PPL will expire and at that point, a NPPL must be applied for.
** You must keep your ratings valid **
There is no requirement at present for non-NPPL holders to submit copies of their medical declarations to the CAA unless they are asked to do so, normally in support of an application for a rating or on request of an authorised officer of the CAA.
IF YOU HAVE A PPL, NPLG LTD DO NOT REQUIRE A COPY OF YOUR MEDICAL DECLARATION