for a professional installation using 15mm copper pipe throughout your looking at around �2500, but depends greatly on what boiler you specify i.e. a heatline condensing combi can be bought for �450 ish whereas a worcester bosch greenstar which we consider to be the best your looking at 900ish so this will obviously push the price up nearer to �3000.
you can specify a particular boiler or your installer will supply one of their choice if you don't specify one.
british gas are notoriously expensive, we had one customer recently who wanted a back boiler removing and replacing with a combi boiler, we did it for about �1500, he then told us b.g. had quoted him �6000.
in any case always get 3 firms in to give you a quote, sort out what type of boiler it will be, what size pipework, position of the radiators etc and go for the middle price, thats the usual advice.
oh and its worth noting that boilers come in different power ratings i.e. 24kw 26 kw etc. even a small boiler will run the central heating no problem, with combi boilers its the hot water thats important, the higher the kw the faster the flow off hot water, some people opt for these cheap heating packages you see in the national papers not realising the boiler is the least powerful and cheapest one and then they are not happy with the hot water output.
another noteworthy comment, be wary of buying boilers off the internet cheaply, as you will really struggle to find someone to install it for you as most installers like to supply and fit the whole job