Screwfix do a basic networking kit for £27
http://www.screwfix.com/prods/10346/...rk-Cabling-Kit
It comes with 50m of Cat 5e UTP cable, four surface mount RJ45 boxes, a punch-down tool and four ready-made 5m patch leads.
You might need to add more boxes/KeystoneJacks (the socket inside the boxes - the boxes supplied can accept a second socket to make a twin).
The same kit can be bought from B&Q for £30
However I would very strongly recommend the additional investment of an RJ45 cable tester - you can find perfectly adequate new ones on ebay for £8 delivered (or £3 if you order from a Hong Kong supplier). The process of wiring RJ45 boxes can be VERY frustrating without a tester!