Unfortunately the CAA's blanket validation for VFR non-commercial flying on an ICAO licence will be restricted to Annexe II aircraft after April 2014. The NPPL will be similarly restricted.
However, if you can get an NPPL medical you can probably get an EASA LAPL medical. AIUI the requirements are very similar, the main difference being that the LAPL medical involves an actual medical examination & not just a doctor's endorsement. Ask on the medical forum or have a chat with your GP.
If you can get an LAPL medical then it would be worth applying for an NPPL so that you can convert it to an LAPL (before April 2015) which will allow you to fly SEP throughout EASA-land. Para 2.3 on p7 of
this document tells you how to get an NPPL SSEA on the back of your FAA licence.
You may be able to convert directly to an LAPL but you will have to ask the CAA. EASA & CAA documents all assume that an ICAO PPL holder wants to convert to an EASA PPL & holds a class 2 medical.