It is quite easy to buy a European sized bed unintentionally. Many stores are now selling European beds to appeal to an expanding market, and a lot of the time it can come as a surprise that your mattress doesn't fit!
If you have fallen into that unfortunate trap, then don't worry. You can use the grid to narrow down your selection and pick the mattress that suits you best
European mattresses are generally 10cm longer than the standard UK size, with a length of 6'6" instead of 6'3". You will also see that there is a difference in width once you start looking above the Small Double variant.