Different people are busy in different cities (and towns); there is no magical place where everybody will earn £££££££ regardless of who they are, what they offer, where they advertise and how they market themselves?

I'd suggest London as a blanket answer, but it's expensive and if you're in Scotland a long way from you (although Easyjet flights between Glasgow and Gatwick are cheap). Have you tried Aberdeen? It's a bit closer, can be lucrative and at least you can get back easily if it's crap.

There's also Newcastle and Carlisle just over the border although I never found either busy - other people have done fine though so sometimes you just have to take a chance?