worcester bosch greenstars and the vaillant ecotec range are the best boilers on the market.
the number of bedrooms in your house though is largely irrelevant as even a low powered combi will run a large heating system. the power of the boiler you buy is dependant on your hot water requirements not your heating i.e. if you want a fast supply of very hot water you need a more powerful boiler.
beware if you buy the 37kw bosch greenstar boiler, thats about as powerful as combis get, your almost certainly going to have to have a completely new gas supply routed to the boiler from your meter, probably 28 or 35mm copper which is going to add a lot of extra expense to the job. in addition such a powerful boiler is going to cost a lot more to buy in the first place and to run.
i would reccomend a vaillant ecotec plus 31 kw which is vaillants mid range boiler and their biggest selling model.