OK, The chap I spoke to at the PFA said that to downgrade to an NPPL, ( which effectively limits you to VFR and UK airspace), all you do get an instructor to sign you off as current and get your GP to sign the medical forms. This means that you believe you meet the medical standards and the GP, on the basis of knowing your medical history, does not disagree.
The NPPL is in Parliament at the moment & will be ready on 28th July, there with be a NPPL Website "next week" and the relevant forms can be downloaded from it. The license takes a few days to process.
Upgrading from NPPL back to CAA requires you do keep current and pass the more stringent JAR Class 2 medical.
Please treat this as second-hand info culled from my scribbled notes and phone the number above for definitive answers.