Finding decent fish and chips in London is surprisingly extraordinarily difficult, so much so that I wouldn't even bother searching it out let alone taking pot luck.
I'm biassed but for fish and chips you want to be in Yorkshire. Also avoid it like the plague in almost every coastal town, no matter which country or county of the UK you're in. The vast majority of coastal chippies paradoxically seem to have the worse fish going.
Not biased at all IMHO! I lived up norf for about a decade, there's no doubt the fish and chips are better.
There was, until about five years ago, a fish and chip shop in Notting Hill/Holland Park borders that was the only one I've ever been to that I recommended around West London. it was a destination place, you'd make a special journey to go there. It was taken over and turned into a "proper" Greek restaurant, they still sold fish and chips, but only because of their past reputation, which regualrs realised pretty quickly was now lost.
If you're unlucky enough to be on Oxford Street, possibly to experience the worst shopping experience on the planet, there is a silver lining. The Golden Hind is nearby, rather an institution in its own right, is probably the best proper fish and chips experience in Central London.
The City/East London, there is Happy Days on Middlesex Street, a street where also daily there is some excellent street food for lunch, mostly ethnic, large portions, we call it coma food, you want to sleep for the rest of the afternoon.