look in your area for charity shops which have a £1clothes rail. But dont buy anything just cos its a quid. If you see something unusual or a good brand name, nip out side and see if a similar item is listed on ebay (make sure you select 'sold' listings when refining your search - this will tell you what the item actually sold for).
I do okay with clothes if theyre a brand name and not run of the mill stuff ..;BEIGE SIZE 12 BLOUSE probably isnt going to get much of a profit. LONG CARDIGAN WITH RED AND BLACK STRIPES... may or not sound delightful to you but you dont find them on every single market stall. I sold one of these for £12 recently, having paid £1 for it. Larger sizes tend to sell quite well.
Barnardos website has a postcode search for your nearest shop - their items in standard shops can be too expensive for reselling but they have discount stores listed too . I have two near me and the items (adult clothing, electricals, homewear etc) are £1.99 at one shop and 99p at the other.
https://www.barnardos.org.uk/shops