At some time during the historical mess we have had to live through since JAA and into EASA, many PPLs were adding an NPPL to their armory in order to benefit from the lessor medical demands of the 'Medical Declaration' should it become necessary. This was almost becoming a standard practice amongst older PPLs, who, at times may have been in and out of holding a class 2. A medical declaration kept them flying whilst class 2 problems were resolved.
From a hazy memory the CAA allowed an SSEA to be added to the UK PPL to obviate the need to have another licence, the NPPL. The SEPL is of course a JAA/EASA invention and i'm sure that at the time it was felt that the UK CAA could not grant an exemption against the need to hold a Class 2 medical. Adding the SSEA to a UK PPL solved this problem.