We would recommend Amazon USA as your first port-of-call. It's a much bigger market.
The UK is significantly more difficult to establish a seller account with Amazon. It's not hard just very slow as they have tight verification processes.
However if you have a product that is somehow very specific to the UK or European market, and you still wish to establish a seller account there, then the principles we teach you are the same, the only difference is where you send your product.