The most common mattress sizes in the UK include:
Single mattresses: 90 x 190 cm (3’ x 6’3”)
Small double mattresses: 120 x 190 cm (4' x 6'3")
Double mattresses: 135 x 190 cm (4’6” x 6’3”)
King mattresses: 150 x 200 cm (5’ x 6’6”)
Super-king mattresses: 180 x 200 cm (6’ x 6’6”)
Emperor mattresses: 200 x 200 cm (6’6” x 6’6”)
Some brands will also have custom sizes available.