Historical version 5 of Category Big People's Shops (view current version)
London is such a big place you can find shops to suit all sizes.
[Rochester Big and Tall]? on Knightsbridge is a purveyor of high quality clothes for big men.
Base Fashions on [Monmouth Street]? is good for ladies.
French and Teague on Great Castle Street near Oxford Circus Station looks pretty good - don't know whether it does men's clothes as well as women's.