I have Telewest (Virgin media) and have an ethernet cable running from the router downstairs up to my office on the first floor.
I think I bought a 20 metre cable, and yes, you can use a standard Cat5 cable with RJ45 connectors at either end.
(You may see Cat6 offered but this is more expensive and a higher quality than you need, Cat 5 will suffice). Make sure you DONT buy a crossover cable.
You can buy cables like this at PC World (cost about �20 maybe), or any online computer store will sell them (Novatech etc)
Or you can buy Micro Mart magazine and find a local computer fair where you can buy cables much cheapers (and many other computer related things cheaper)
http://www.micromart.co.uk/