I understand the CAA is at an advanced stage of producing the 'conversion report' required for holders of a NPPL (A) (SSEA) to a LAPL (A).
My reading of the likely outcome is that the holder of a NPPL (A) (SSEA) will be able to obtain a LAPL (A) providing:
(a) He holds a LAPL medical certificate in accordance with Part MED which is due to be final in legal terms in the next few weeks (this certificate can be issued by a GP but is slightly different in terms of process to the current GP counter-signed self declaration)
(b) Fulfil language proficiency requirements in Part FCL to level 4 English (unless already holding a UK Flight Radiotelephony Operator's Licence)
(c) Min 6 hours since obtaining NPPL (A)
(d) Recency requirement per EASA FCL 140.A(a)
(e) Pay the appropriate fee to the CAA
Most NPPL(A) holders should meet these without problem (money apart!).
There are variations on this theme for SLMG ratings etc for LAPL (TMG)