In uk.transport.london David Horne, _the_ chancellor (*)
wrote:
www.nationalexpress.com will allow you to buy a ticket from abroad- and
even though they don't run that route, they will sell tickets on it.

Make sure you get a train ticket, not a bus ticket! (They do both).
http://www.nationalexpresseastcoast.com/ is the train booking site.

I don't think they'll post you the tickets, but you can select to collect
them from a machine at the station. Take the credit card you used to book
them with you as you'll probably need to use that in the machine.

Theo