An SSEA Class Rating certainly may be included in a UK PPL - see the ANO!
This came about many years ago. The idea being that it if a PPL pilot was no longer able to hold a Class2 medical, he/she could then fly within SSEA limitations provided that he/she could make an NPPL medical declaration, without needing to apply for an NPPL. But to continue to do so after the extant SEP Class Rating expiry date, the Rating would need to be reissued as an SSEA Class Rating included in the PPL.