Mash is the best fishmonger locally by a long way. They have been in Brixton forever.
The produce is top notch and they will prep it and tell you how to cook it.
I regularly buy their sardines when in season, 8 for under a fiver. All filleted and ready to throw in a pan.
No supermarket can match their expertise and knowledge.
Brixton newcomers note - use it or loose it.
L S Mash & Sons supply a range of fresh and frozen fish.