Enemy messages were obtained by the Y Service, a chain of wireless intercept stations...
… across Britain and in a number of countries overseas.
Thousands of wireless operators, many of them civilians but also Wrens, WAAF personnel and members of the ATS, tracked the enemy radio nets up and down the dial, carefully logging every letter or figure. The messages were then sent back to Bletchley Park (Station X) to be deciphered, translated and fitted together like a gigantic jigsaw puzzle to produce as complete a picture as possible of what the enemy was doing.The main Royal Navy sites were at Scarborough and Flowerdown, near Winchester, the army site was at Chatham in Kent and the RAF site was at Cheadle in Staffordshire. A number of new sites were set up around the country at the outbreak of war. There were also several British intercept sites overseas at Malta, Cairo, and Sarafand in Palestine, and for Japanese traffic at Abbottabad and Delhi in India, at Singapore, and later at Colombo and Mombasa. These intercept stations all worked closely with Bletchley Park.
There were also intercept operators based on ships and mobile army intercept units with British forces on the ground in the various theatres of war around the world. A lot of intelligence could be worked out from what the radio operators were saying to each other and their locations could be tracked down using radio direction-finding equipment. But most of the important messages were in code or cipher and had to be decrypted.