I've used British Gas for years and am very happy. Not sure how they compare price-wise, but they service the boiler annually as part of the deal which gets some of the money back anyway.
We had to call them recently - we had a new boiler installed, the plumber (not BG) was an idiot without a clue - we had him back a few times, went through the whole charade of "it's meant to do that" "no, I think I'll try this" etc etc then he sent his own electrician who was just as useless, then just stopped returning our calls. This lasted over 6 months. Called BG - hey preston, one fully working boiler within a week - and no extra charge. Even though I explained the situation, and none of the problems were their fault, they still sorted us out! Can't speak highly enough.