A little bit late now but try www.thetrain.com (or .co.uk) It's something to do with Richard Branson and Virgin Trains. You should be able to get some fares there (cheapest for advance purchase).

Might be worth checking out www.easyjet.com and the Go site for flights to Scotland (might save some time and might be cheaper than the train).

Since you're staying on the weekend, take a peek into www.travelweb.com for the weekend specials. There are many rates for London and Scotland this weekend,