My friend is a plumber and, because he works with gas central heating, on the Gas Safe Register.
Simply look for a plumbing and heating business with the yellow triangle or, even easier, search here:
https://www.gassaferegister.co.uk/find-an-engineer/
(You might not actually need a gas engineer anyway, as your description of the work suggests that he/she won't need to go anywhere near to the boiler but it might still be best to employ someone with the relevant qualifications just in case he finds that he needs to make adjustments to your boiler).