DW, I did this when we bought the house we are currently in as it needed a complete rewire. The course cost about £700 IIRC and lasted two weeks. At the end of it I sat an exam over two days which involved wiring numerous circuits which the examiner tested for faults and then sitting a (stupidly easy) multi-choice paper. For that I got a certificate that I presented to the local building control officer who confirmed that I could then sign off my own work.
Getting the BCO to agree with this sign off route was a bit of a pain in the proverbial, he admitted up front that it was a perfectly legal route he'd just never come across anyone that wanted to actually do it before so he disappeared for a week while came up with a new internal system just for me
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Of course, once you have your certificate you still have to cough up £50 for each job you do as you have to file a small works notice but a job can be literally any amount of wiring. Also, strictly speaking, you need to have a properly calibrated meter in order to perform the tests necessary needed to sign off. I have such a meter, made by Dialog, but it wasn't cheap. If you are wiring your whole house it's probably worth getting a meter as they hold their value pretty well, for a single job you'd really need to borrow one.
You can't really work on anyone else's house since you probably wouldn't get insurance unless you joined once of the registered bodies which cost a fair bit per year. Finally, if after wiring everything up you aren't confident in your circuit you can always get a sparky in to do a test and inspect (you can't do a test and inspect on a Part P certificate) but that will obviously cost you.
Would I do it again? Probably, I've saved literally thousands wiring the house myself. The biggest down side actually is having to listen to all the whining busy bodies who for whatever reason want to tell you that you need to be a fully qualified sparky to do simple domestic wiring.
P.S. you also can't do three phase on a part p competent cert.