Walthamstow Market in North East London is said to be the longest street market in Europe, it stretches all the way down the High Street and you can visit Tuesday to Saturday. I visit frequently and have many favorite shops that I have been visiting for years, most of the bricks & mortar shops are open 7 days a week but please check.
If you are not familiar with the area it is easy to reach either by tube or the Underground to Walthamstow Central, the overground runs from Liverpool St., for the Underground take the Victoria line.
I usually take the train to Walthamstow Central, walk down the whole market and jump back on the train at James Street at the bottom of the High Street.
Some of my favourite shops which are featured in this video are:

Zayan Buttons 138 High Street E17 7JS

Fabric Textile Central 134 High Street E17 7BS https://www.fabrictextilecentral.com

The outdoor trims stall I feature is usually on the left as you continue to walk down the High Street in the direction of James St.

Saeeds Fabrics 9496 High Street E17 7JY
https://saeedsfabrics.co.uk

Fabric Store 2 70 High Street E17 7LD

New Fabric Shop 91 High Street E17 7DB

Ribbons 89 High Street E17 7DB

Istanbul Fabrics 175 High Street E17 7BX