It's other signals interfering that's the problem
The signals given off by small radio mics buried in a luvvies' wig aren't that strong, which means that if someone else down the road is using the same frequencies then there's going to be interference. See 'Spinal Tap' for an example of what happens. The entertainment industry therefore needs it's own nationwide frequencies to be sure that there won't be any problems.
It's not just theatre that's affected it's also rock'n'roll, and the massive conference and events industry. Everything from the lighting up of the London Eye at New Years to those crazy lighting pantographs used in the last Coldplay tour relies on radio control.
Britain is an international leader in such things; Cirque du Soleil rely on a British company to build their automated set pieces and there was a heavy British involvement in the Asian Games opening, the Turin Winter Olympics opening and the Athens Olympics opening. Yet despite this OFCOM seem determined to undermine the industry. Similar threats against the financial sector would be short-lived, it's a shame the government don't care about the technical side of the entertainment industry.