In order to exercise LAPL privileges, you must re-grade your Part-FCL PPL to a Part-FCL LAPL using
http://publicapps.caa.co.uk/docs/33/...e04enabled.pdf and paying the relevant fee. You cannot simply use a PPL restricted to LAPL-level privileges with a LAPL medical certificate.
The CAA has also introduced some totally baffling 'self-certification' medical scheme, but the legality of using it for anything other than for flying non-EASA aircraft on a legacy NPPL or UK PPL (not a JAR-FCL PPL) is open to doubt - even the CAA isn't sure...
In your case, I'd recommend you forget about it and just take the LAPL route.