While deciding on the "best" is a matter of personal taste and style, London certainly has options for finding heavy metal T-shirts. A good place to start would be the Camden Market.
Camden Market actually refers to several markets in the Camden Town borough of inner-city London. Bush Street, Camden Lock and the Lock Village offer a large variety of clothing merchants and vintage dealers. The Inverness Street Market is an especially notable area to find vintage clothing and rock n' roll items.
As of 2014, other areas of London also offer street shopping, such as the Hyde Park and Soho districts. Many T-shirt stores line the streets of London, and classic heavy metal shirts are sure to be found.