There are a number of formation websites. (ie http://www.companiesmadesimple.com). I was wondering why you would use one of them rather than just using the companies house website?
(I'm based in the UK)
There are two parts to forming a Ltd company in the UK
The Articles of Incorporation are the documents that describe the company legal structure. For most 'simple' Ltd companies these follow a fairly standard format and there are many many sources to get boiler-plate documents where you just fill in the blanks. For something more complex you can get custom ones written up (for a fairly heft lawyers fee)
A) Can be found all over the place - the website you listed, WH Smiths book section and a gazillion others.
B) Can only be done at Companies House.
I don't believe that there is any advantage of doing A) at Companies House. Its something that they have only fairly recently started doing in partnership with another org (BusinessLink) and I suspect its just an attempt to 'cash in' on passing traffic - their position would be untenable if there were any favourable terms.