What happens to all the male chickens in UK as there is only one rooster per breeding chicken farm?

This isn't entirely accurate. Most commercial egg-laying operations have one rooster or sexed female hen per 10-12 hens because it is not always possible to determine the gender of a chick until it is at least six weeks old, so an accurate 1:10-12 ratio can't be guaranteed from hatch. Any surplus male chicks are destroyed shortly after hatching in various ways, such as maceration or gassing.

In the UK (and much of Europe), there is a push to end this process of chick culling and several companies have invested in research to develop technologies to determine the sex of a chick before hatching.