You fill out form SRG\1102 (downloadable at
http://www.caa.co.uk/docs/33/FORSRG1102.PDF), then send it with a copy of your Medical Certiicate to the CAA.
Plus, of course Form SRG\1187 and payment for (currently) £63...
And the CAA's original RIA claimed that maintaining a JAR-FCL PPL(A) would be cheaper than maintaining a UK (lifetime) PPL.......