If you are trying to buy MDA (i.e. from the Hauptwerk web site "Store") you need to select EU and proceed to purchase the item(s) you want and when you provide your address the United Kingdom appears as one of the countries listed (alphabetically).
You can buy from EU based resellers (full list at https://www.hauptwerk.com/store/resellers/).
If you are trying to buy a sample set from one of the sample set producers... which one?
Iain