Definitely do not get it from BG, massively overcharge you. Our old boiler died so went through the home insurance over and they quoted us £2400 for a top of he range Worcester Bosch. Went to an independant and got the exact same boiler (Worcester Bosch Greenstar 29 CDI) brand new and fully fitted for £1680 including 5 year warranty.
I guess the only advantage of getting it from a big company is that they offer payment plans so you don't have to stump up all the cash in one go. We were lucky family we're able to lend us some cash.
To to add to that if you get a perceived "lower brand" you could probably get it for a few hundred cheaper and a lot of these come with 10 year warrantys. Definitely worth shopping around.