try www.thetrainline.com - it's what we use ourselves over here! - GNER is totally unreliable (and so are their trains). If you want to spoil yourself, why not do the Caledonian sleeper... never done it myself but supposed to be v fun.

To find affordable places to stay in London, check out either www.lastminute.com or better still, www.octopustravel.com - you can find good deals on there and the info is reliable. Plus they give total price of the stay in dollars...