Ive just ordered three windows and a back door for less than £800 delivered and inc VAT, so £700 will be £350 plus their mark up, often the seller buys from someone else, mine have been bought from a company in Surrey but I have found out they buy from a factory in Kent that doesnt deal with the public or builders only glazing companies
I had three prices
£800 inc vat (accepted credit cards)
£940 no vat/cash only*
£1100 no vat/cash only*
* no warrenty either probably
All using the same profiles, so it pays to shop around