I dread to think what British Gas has quoted. Guessing £4k plus for a straight swap?
A new sedbuk band 'A' gas boiler, either system (mains pressure) or gravity should cost you around £1,300 and £200 for a new flue (if it's needed) for a 24(ish)kW. Any decent gas registered plumber would replace it in a day easily. They do tend to want to make good money on boiler changes, because they can, but £500-£700 should cover the install.
I would expect to pay no more than £2,500 including VAT.