Part P, (or more correctly Domestic Installer) on its own is useful, its mainly building regs with some hands on electrical work and testing. However to be able to use it and self certify your electrical work you will need to register with a competent person scheme (ELECSA look good). I reakon its worth it as many Plumbing jobs need wiring eg a bathroom new lights, extractor and electric shower. My course was about £750 and looking to register with Elecsa soon.
Downside - test kit is around £700, but I reakon it would limit what you did if you couldn't self certify!