As you say Miltons is a good shout. There's also Boodles if you fancy checking out some Patek goodies, it's at the front of the Grosvenor shopping center.
Aside from that you have Mappin and Webb for Rolex, if you like being sneered at by store managers called "Gary".
There's also a Beaverbrooks, Chisholm Hunter, Ernest Jones, all found on, or just near Eastgate street (main high street).