It depends on the number of bathrooms, Hot water usage and heat loss ( The U value ) of you're Home. It sounds from you're breif description that a solar condensing system boiler with an unvented cylinder would be you're best option. Most effective, green and expensive to install and buy though they say you will re'coupe you're extra money by the cheap running costs though.
Alternativeley a solar condensing combination boiler would be you're next cheapest option this does away with the need for a hot water cylinder and relevant pipework ( may even give you room for a shower where the airing cupboard is ???
The cheapest and most popular option of all is a condensing combi boiler
Good makes are Worcester/Bosch, Ideal and Baxi.
Makes to avoid are Ravenheat, Biasi, and although this will probably be un-popular dare i say Vokera boilers to avoid aswell !!! The amount of broken down Vokera's I have to deal with is around 6/10 of every breakdown.