My daughter had British Gas some years ago and they were really good, but today it seems they have learnt from their errors and now they are more careful on what they cover.
Father-in-law had a company set up by brother-in-law after his heating went wrong, travelling 100 miles to repair simple faults was not really on, but they wanted to do so much before they would take over the maintenance, they tested water and said it needed a flush first, we had this done, not by them though, and then they came up with another excuse, this went on for around 6 months, then father-in-law died, and they still had not been satisfied to take on the system.
But still wanted us to pay, saying the cover would be passed to new owners, who on buying the house had ripped out the old system and replaced, so clearly did not want cover for a system which no longer existed, we did in the end get the money back.
The use of a local boiler man does seem to work, they do the service, and will respond when there is a fault, but a lot of companies caught a cold with overall cover where people took out cover when there was a known fault, waited a few weeks then claimed, as my daughter did with British Gas all those years ago, and if you consider the cost of a service, which I seem to remember last one cost me around £125 it has to cover that to start with, so has to be at least £200 per year, where I will get boiler serviced around once every 3 years.
It is good for rental property, so you get the gas safe certificates every year, but for private property does not really work out.