Hi,
You sure can restrict the countries available in the country selector by disabling the country zones in the menu System>Zones. However, in that case, the users won't be able to select their country in their billing address either, even for downloadable goods which you want to be able to sell worldwide.
So I don't think that it's the way to go.
Here is what I propose:
- make sure that the "force shipping regardless of weight" setting is turned off in the HikaShop configuration. That way, the shipping address, and shipping method selection will only appear on the checkout for products with a weight.
- make sure that the downloadable products don't have a weight and that other products have a weight.
- create shipping method(s) with their "zone" setting restricted to the UK country zone (no need to create shipping zones in your case). That way, when the shipping methods selection appears (for shippable goods), it will only display the shipping methods if the user shipping address is in the UK, and otherwise it will display an error message that no shipping method is available for the location of the user and will prevent him from checking out.