Many of the builders merchants are a bit coy about publishing their prices, partly because of their obscure discount policies but here are a few price guides for building materials from suppliers on the internet. This may help with pricing up smaller jobs. With larger jobs you may be able to negotiate discounts.
With most types of domestic self build you can claim back the VAT on building materials at the end of the job. However there are several exceptions (such as a granny flat) and it pays to read the HM Revenue and Customs guidance notes on this.
Buildbase - broad range. You need to enter your postcode and select a branch
gibbs and dandy – Landscaping products
Fulham Timber Merchants Ltd. - Timber and other building supplies
Drayson Timber - mainly timber, boards and ironmongery
Hallgate Timber - sawn and planed timber
Chiltern Timber - mostly timber and sheet
Pagets - mostly Yorkshire Stone