You can go to the best bespoke shoemakers in the world in London, and get something made just for you ... more importantly, just for your feet ... but you will pay a pretty penny.
Is that what you are after, or are you wondering if Marks & Spencer is a good place to pick up some decent shoes on your next visit??
Many quality brands of bench made shoes to be had over here mate. My favourite shoemaker is Trickers which just happen to have a shop in Jermyn Street near all the shaving shops! Hope your credit card is ready for a good workout!
Other quality shoes to look out for are John Lobb, Church, Cheaney, Alfred Sargent, Crockett & Jones (mentioned above), Grenson, Loake, Barker. I have no idea about shoe shops in London as I avoid the place as much as possible! Sure you'll find plenty of decent shops selling quality shoes though.
London is definitely a place where you can find the best of practically anything if you're prepared to pay for it. I'd check out the prices of Harrods vs standard rrp. If they don't mark-up too much, you should be able to choose between many brands there without walking around for hours and hours.