Not sure if those figures are accurate but it may have to do with not all fishing ground being made equal. British waters are fed by the gulf stream bringing with it rich supplies of nutrients and migratory fish. It's not for no reason that everybody else wants to fish in British water.

Go to your local fishmongers and check where the fish comes from. Norway and Spain may be among the typical countries. But this only means the trawlers that brought it in are registered to those countries, and has little bearing on the actual location the fish were netted. If Britain were catching 30% of fish you would see much more British fish on supermarket shelves, even in Switzerland.