OP I don't sell a physical product, but I am self employed and provide a service. Forgive me if some of this is not transferable, but I think a lot of it could be.
I accept that I'm responsible for finding my clients/generating my income so I need to invest some time and money every week into making sure clients find me.
You could have the most amazing product in the world and if only the same few people know about it you aren't going to get anywhere. There are plenty of people in my profession who don't make it work as their main source of income and it's not because they're crap, it's because they are intimidated by/don't bother with marketing
I have tried a whole host of things, leaflet drops, trade directories, social media, my own website etc. I see it all as a big experiment. If one avenue isn't very productive I don't do it again. You just want to make your product as visible as possible.
For me my own website has probably been the most successful, but it was a bit of an investment and i worked hard on my SEO so that if you type "Xxx in my area" my website comes up first.
I'm also listed on Google business and bing businesses.
If you type your product into a search engine what comes up first? Are you anywhere there/on those sites? Can you get there?
Any way you can get your product more visible is helpful. Every 6 months or so I pay for a business coach to cast their eye over things. It costs less than £50 but they suggest tweaks and it can make a real difference.
It sounds like being handmade, a UK business etc is potentially a bit of a USP for you if you're up against the likes of shein etc so I would lean really heavily into this when talking about your product. As pp said maybe put a little union jack on your listing pics. Make sure you mention "handmade" "bespoke" "small UK business" "crafted with care in the UK" etc lots.
Why should people buy yoir product over the alternatives? Tell them.