I inherited British Gas 3-Star cover from a previous houseowner and, in the main, have been pretty good for the gas CH side of things. Maybe not cheaper than getting each individual job fixed, but I guess pretty close and allows you to spread it more.
However, on the occasions I have needed their household appliance cover, they have been f***ing useless every time. Apparently having LPG CH but an electric oven or dishwasher is just too complicated a situation for their computer to handle, and they cock it up every time.
But the CH cover is good...