There are over 80 Michelin Star places in London. Not a small number of restaurants. Not to mention hundreds more that aspire to get a star and care deeply about being Michelin-level quality.

Visiting Billingsgate will teach you that quite a few restaurant folks (and quite a few "normal people") shop there, in addition to (smaller) fish traders.

Smithfield, at least to me, always had a different vibe; more pro-to-pro/business-to-business, but also they deliver or commission to pre order there; different from Billingsgate.