The cot mattress size most used in the UK is the size 140 x 70 cm this size is sold in large quantitied in both solid foam core and spring interior cot mattress types, it is used in many cotbeds by most of the well known manufacturers.
As the 140 x 70cm size has a tolerence by british standards on width and Length this tolerence is that the gap must not be more than 3cm, you will see that some manufacturers such as Mamas and Papas size 400 specify a mattress size of 139 x 69cm where in reality a mattress 140 x 70 will be well in tolerence to fit the same cotbed, the reason for calling the mattress 139 x 69 is so people only look for those dimensions and will only see the branded mattress types,
Other Manufacturers call their mattresses 140 x 69cm for the same reason as above. the old imperial size for the 140 x 70cm cotbed mattress is 55 x 27 inches. you may see this size stated on some older cot sizes.