As a manufacturer I recently read a review regarding UK bed sizes, the writer gave an inaccurate sizeing so here are the correct sizes. You should also be aware that due to the manufacturing process a tolerance of 3.8cm or 1.5" should be made. The sizes listed below are the spring size not the finished product, so if you purchase a mattress with generous layers of filling the thicker the mattress becomes, hence the tolerance.   

Small Single 75cm x 190 cm or 2ft 6" x 6ft 3"

Single 90 x 190 cm or 3ft x 6ft 3"

Small Double 120 cm x 190 cm or 4ft x 6ft 3"

Double 135 cm x 190 cm or 4ft 6" x 6ft 3"

Kingsize 150 cm x 200cm or 5ft x 6ft 6" 

Superking 180 cm x 200 cm or 6ft x 6ft 6"

Queen size is an American term for Double. You should also be aware when purchasing a bed from Ikea only their own mattresses will fit their beds, Ikea sizeing is not standard so do not expect to buy  a bed from Ikea and a mattress from elsewhere as it will not fit.