I *think* if I buy pre wired 12v halogens that plug and play into a transformer then I am just wiring a transformer to an existing switched ceiling fitting and can therefore do it myself without part p notification. Am I right? Or does the bathroom bit mean I can't do anything??
Zone 1 and I'd use appropriate jetproof fittings obviously.

These shrouded halogen fittings give off so little light it's not true. Why is there still a craze for halogen downlighters that require 100's of Watts just to light a bathroom????
Something that is surface mounted and can reflect light off the ceiling is vastly more efficient!
Not the answer to your question, I know: Sorry!

It's a bit of a grey area as it's in a "Special Location" meaning it has a bath or a shower.

YOu could read it as your only changing the ceiling rose???

"Replacing sockets, switches or ceiling roses (even when in a kitchen, bathroom or ‘special location’). Providing it does not include the provision of any new fixed cabling"