How do I change which regions I want to sell in?

I believe the European countries are automatically included in a UK account, as Amazon considers the UK to be part of the European region (which it is).

To avoid selling outside the UK, go to your Account Info (cog icon, top right of Seller Central) and look for Listings Status / Going on Holiday? Then set each foreign site to “inactive”.

Next, go to Shipping Settings (also via the cog icon) and alter your shipping template to exclude everything except the UK.