What would you say is the best street markets in London? Why?

No way to answer unless you tell us what sorts of thing you look/shop for. Each market has a different focus.

Plus it depends on which days you have free since some are on specific days o the week.

I just returned from the Portobello Road Saturday market and it was great.
Portobello Road Market

On the way back, I asked the taxi driver what was going on at Trafalgar square and he said it was a Sikh festival.
Here's more info -
https://www.london.gov.uk/events/201.../vaisakhi-2018

I don't know what is "best" but I really enjoyed Portobello and check to see what special events may be going on while you are here.

Brick Lane (despite my not knowing much about London markets). It is completely eclectic with food from all over the globe, live music and shopping stalls. A great melting pot!

On a Sunday there are several markets close together in East London - not just Brick Lane, but Spitalfields nearby, and a little further, Columbia Road flower market, and in the other direction Petticoat Lane. But they're general markets - between them, bric-a-brac, arts and crafts, clothing and household goods. If you're looking for anything like a farmer's market, you'd best try Borough Market.

I wasn't really looking for myself. Just was interested in other people's opinions.
I like a variety of foods and shop for a lot of different things.
Thanks for the replies.

Oh - I figured you were asking for yourself since you are going to the UK. If you just mean markets 'in general' -

Borough Market for sure (but not on Friday afternoons or Saturdays because it gets sooooo crowded).

Portobello Rd - Saturdays early. Gets VERY crowded by mid morning.

Columbia Road https://www.google.com/search?q=columbia+road+flower+market&client=safari &channel=iphone_bm&source=lnms&tbm=isch&sa=X&ved=0 ahUKEwij8rzeid7aAhUJFXwKHbAMB9sQ_AUICygC&biw=1385& bih=729

Spitalfields

Camden for sure

Leather Lane

(Have you worked on making your itinerary work/doable? )