The sustrans site is good for showing you specific routes but these routes are really just linked together roads which you could put together yourself. They aren't cycle specific usually. When I first started cycling I followed a couple of signposted routes and soon realised that the roads are a patchwork of little roads and if i didn't stick to the signposted route i could do a very similar route with other little roads which would be just as good. The best way to plan a route for me is to just look at google maps and see if you can string together roads from A to B without using the green or blue roads which are major roads that you either are not allowed onto or you wouldn't want to ride unless you have a death wish.