There is no such thing as a CAA PPL, only JAA.
Have a look at LASORS on
www.caa.co.uk as it should answer your questions.
An MEP rating requires post PPL experience, so you need to assess if you have this. An MEP is for any multi engine piston aircraft below 5700kg, with some exceptions, and subject to differences training. A King Air does not fit this category as it is a turboprop, so type rating required.