Liverpool would scan easier - "big London Town" is a bit clumsy, and Liverpool Town would seem more natural. So maybe there's some actual historical basis for the Master being in London.

He did get taken to meet Queen Victoria in Windsor, though that was after his victory. And there's a row of houses in Walthamstow called "Master McGrath Terrace" (in Shernhall Street).

Here's a site with a biography and a couple of pictures.