Have used British Gas for years for boiler cover contracts including annual service. Have always found them very good.
I only choose cover for Boiler and controls, not the full system (can sort that myself) this year the annual cost was £201 on a 4 year old combi boiler. They do tend to increase the price annually. This year they tried a renewal for £240 said I would cancel so they reduced it to the £201.
I think it would be difficult to find a local engineer that would offer a breakdown & service contract
including parts, due to the cost of some components and the huge variable stock they would need to hold. Additionally they would probably struggle to maintain an acceptable breakdown attendance during busy times.
Shop around and try a bit of negotiation. It costs nothing to ask