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 from suppliers.
Thompson Building Supplies. Mostly heating and plumbing
gibbs and dandy. Landscaping products
C & W Berry Ltd. Timber and builders merchants. Hardware, plumbing, electricals, ironmongery etc
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
buildingmaterials.co.uk wide range of building materials