I live near Newcastle upon Tyne and unfortunately there aren't any "guinea-pig/small pet" specific stores, though I know that it's very unlikely US pig owners have them either. You can find fleece sellers in craft markets / children's blankets if you want to actually see it before you buy it, and there's always the option of making your own accessories if online shops irritate you too much. I point-blank refuse to visit pet stores as, to be honest, the only "accessories" they tend to sell are completely inappropriate for guinea pigs, such as running balls and fattening seed sticks. I am terrible at sewing and source all of my things online: if they're not what I want, I send them back, simple as! It is frustrating that there's not an adorable little shop filled with fun and interesting things nearby, but there are a tonne of them online, so it's the next best thing.
Here are a few of the websites I love:
For accessories:
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Amazon is brilliant for hiding houses etc, and the review system is helpful.
For Fleece:
https://www.abakhan.co.uk/ Ordered by the metre and returnable provided you don't start cutting it up etc before you send it back.
https://www.myfabrics.co.uk/fleece-fabrics.html
Don't forget crafts markets/ASDA/Tesco's/Amazon for this too.
For (premade) Cages:
https://www.candcguineapigcages.co.uk/ - The UK equivalent of the cages store here.
Ebay sells premade CC cages too. If you want to completely customize, try B&Q, Wilkinsons, or a few of the links already mentioned on this forum.
Hope this helps!