It’s cool that you’ve gotten a few sales. Not sure about your ads, but I would imagine your website is a part of the bottleneck. It’s very clinical and doesn’t really “sell” aside from showing a bunch of listings.
The supplement game is super competitive, and while you may have an advantage in that you can mainly target UK traffic, most people can still jump on Google and find the same supplements at around the same price. That means it will come down to branding and UX.
I’d recommend leaning into your brand image, talk about yourselves, why you got into selling supplements, your mission, etc. Also, think about who is shopping for your stuff, and most importantly, why they would buy from you and not someone else.