When buying from Amazon UK and shipping to Malta (or any other country except the UK itself), you will be given the option to pay in GBP or your local currency (EUR in Malta’s case).
By default EUR will be selected, so make sure you switch that to GBP. Amazon uses an exchange rate that is worse than the one offered by your local bank, and you would, therefore, be losing money if you pay in EUR.
I then make the payment using my TransferWise Borderless Mastercard debit card, as it automatically takes my Euros and converts them to GBP to make the payment at the best rates. Not only that, but I can also convert Euro to GBP at any point in time and store them in the Borderless account. Then when I pay on Amazon UK it will use the funds in the GBP account I have.
See also: N26 vs Revolut
When traveling abroad you will encounter this choice when paying at shops and restaurants as well, and even in those cases, you should always pay in the currency of that country by using a card like N26 or Revolut. The amount saved really adds up over time especially if you are making larger purchases from Amazon.