Nick here from Shopify.
I'm not completely sure I understand everything you said. You can definitely add a .co.uk subdomain and then if you wish to make it your primary domain and you can definitely charge people in £'s. If you are using Shopify Payments then you can apply the Multi-Currency feature to do this. If you are not using Shopify Payments you can only charge one currency and would need to change your primary currency to do that if you aren't already charging in £'s.
Can you let me know where you are currently based and what currency you are charging in now and we can take it from there?