Stop before you go any further !!....and you need to learn a lot before you proceed. If you plan to buy new goods to sell on for profit then you ARE a business, and you need to be registered as such with HMRC as self-employed (even if you also have other paid employment) - probably sole-trader, even if this is just a 'side-hustle'. You also MUST have an Amazon BUSINESS seller account, not individual account.

There is a VAT threshold for UK residents - it is turnover of £90,000, BUT if you do NOT live in UK but wish to store your goods here in UK for FBA then you MUST be registered for VAT here.

Suggest you go and read Amazon's Seller University - under Learn on your seller homepage, and whatever you do do not buy your stock and send to Amazon until your account is fully verified - which can take a while at the moment, or you will be charged storage fees, and make sure you understand all the costs associated with FBA., and don't listen to all the 'get rich quick' videos on Youtube - can be very misleading.