Any decent solicitor would have advised you to have a separate survey done on the central heating system - it is relatively cheap and avoids this sort of situation.
Unless the seller has blatantly lied on the property information form you may have no recourse against them. The boiler may well have been regularly maintained and it was presumably during such routine maintenance that British Gas advised that they 'need a new boiler'. Was it not working at all? If not then you are probably in a stronger position, but if it was in working order, despite having been advised that it should be replaced, there may be little you can do.
I have a maintenance contract with British Gas, and as part of the contract they come out once a year to check the boiler. on the first such annual inspection, the engineer told me I should consider replacing my combi-boiler, which was four years old at the time ....... I have had no problems whatsoever with it since that time five years ago! I think they are briefed to try to sell new boilers!