You trust the marketing content of a company website?
Probably not but what precentage of total customers do those reviews represent and what is the precentage of bad reviews?
Why? Small local firm has a reputation to maintain or they'll go out of business as the word gets around. Any guarantee of that duration isn't worth the paper it is written on unless insurance backed, no matter the size of the company. Look at how many apparently succesful big companies have gone bust recently. Anglian and Everest will have spent a lot of money on lawyers writting the small print of the "gaurantee" to minimise any claims. It may even tie you into an annual "maintence check" that has to be done by a a "qualified" entity at £50/window or similar catch...
Define "massively". Ignore any list prices from Anglian/Everest they are just a sales scam. As a guess I'd expect a good local installer to be 20 to 50% below A or E.
Local firm relies on reputation and word of mouth to stay in business. Anglian/Everest spend a fortune on ads... Having has double galzing installed by Anglian and a local firm you really, really, do not want to use Anglian and Everest are in the same camp as far as sales pitch is concerned. I did not like the abusive response I got when I phoned to reject an Everest quote or the follow up calls trying to make me change my mind.