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Yeah, my local council (harringey) sub everything out to a local contractor so £325 a time for a certificate. When thorough 16 and 17 edition qualifications, through ex employers so that I could identify that work was done correctly and was correctly certified. Suppose that is what you need when you employ the cheapest bidder and they then sub out to the cheapest person.
lynx-
The BS7671 qualifications are pretty useful but you need the Installation and Testing one 2391 to cover basic installation. The 2392 is what you need if you are carrying out condition reports.
It's not that easy to get registered and stay registered if you're not working on consumer unit installs etc. regularly. I couldn't see that it was worth it for me as I'm not interested in doing that kind of work. Staying registered just to be able to add circuits or work in gardens is tempting.
Airhead
The cost does depend on your local council. I could probably justify it on my own place because the labour would be more than the certificate. If you have the qualifications it's tempting to forgo the bullshit and easier to ask for forgiveness.