Unfortunately you will be mainly reliant on the person who is fitting it to advise you as they will have experience of certain makes and have their own preference for installation.
You should make sure that they are registered with CORGI and that you know eactly what your options are for a boiler that suits your home needs. They should be able to recommend (and quote for) a few so that you can look up details and reviews on the internet. My boilerman told me the kw I needed and suggested a few makes and I opted for a Vaillant (longerg guarantee), other fitters would prefer a Saunier Duval, a Biasi or a Worcester, there are so many to choose from, so you need proper one-2-one advice for your home.
There is some information here, but it is primarily impartial.
http://www.nef.org.uk/energyadvice/boilers.htm
http://www.centralheating.co.uk/index/fuseacti on/directory.memberList/con_id/5367