Re: Scenario: in April next year, Joe Public wants to
Assumption: keep it simple, ignore Amazon marketplace, ignore "sold by xyz fulfilled by Amazon", assume amazon.co.uk and England/Wales consumer contracts and all the usual reasonable assumptions. Also bear in mind that I am not a lawyer, although far too many politicians and their friends are lawyers.
Begin at amazon.co.uk.
Find (if you can [1]) the Conditions of Sale that apply today. Optionally, also find those that applied in the previous version (which appears to date from 2015?). Have a look at (for example) the section headed "Applicable Law".
Like I say, I am not a lawyer, but as far as I can see, Amazon's purported contract of sale today for a UK consumer buying something sold and fulfilled by Amazon would at first appear to be between the consumer and Amazon EU SARL (who would appear to want to apply Luxembourg law).
If Amazon's not your thing, think about any of your favourite trans-European outfits. Apple, Costa, whatever floats your boat (come to think of it, why not British Airways, or Santander, or whatever).
[1] If you can't find it, I sympathise. Try starting from here:
https://www.amazon.co.uk/gp/help/customer/display.html?nodeId=1040616
But beware of the leopard.