Replacing a gas boiler fan - UK gas safe regulations
You can work on the boiler if you are Gas Safe (oka Corgi) registered.
Sadly, even as a professional engineer who may well understand gasses, reactions etc much better than a registered installer, you don't qualify as a competent person...
Some registered people may be prepared to allow you to do bits and sign off on them after they have inspected - but they are about as rare as hen's teeth... (and yes my neighbour was registered and did that for me...)