If you own a domain, you can create subdomains and assign them to different sites. However, if you wish to have different settings for your UK customers and for your other international customers, you would need to have two separate Shopify stores. You could then assign different subdomains to each one.
Please also be aware that Shopify has recently released a feature that allows merchants to set prices to be tax-inclusive or exclusive based on the customer's location. This means if you set your prices to be tax-inclusive on your site as you're selling into the UK, customers where they do not need to pay tax will see the tax-exclusive price at checkout.
Victor | Shopify Social Care
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