Most major cities have places that sell albums so when someone asks me for a K-Pop store I try to think of stores that also carry a good selection of merch like posters and such but sadly I can't recall any in London, I do remember visiting a few stores but none that really stood out. I did a quick google search and many people recommend Sokollab and looking at the pictures I think it was one that I visited, mostly albums but might be worth checking out!
On a somewhat related note i looked for a Korean restaurant when I was there and when I entered they had a sign that said Jennie (Blackpink) ate there after one of their concerts, they had her signature and a bunch of K-Pop stuff on the walls so if you like Blackpink it might be worth it to stop by. I had the Jjajangmyeon which was good and the place had an authentic feel, it's a small restaurant located in the basement of a Korean food store so you will have to enter the store and go down the stairs in the back.
It's called
WooJung (google maps link).