There ought to be laws against monopolies in the Union movement just as there are laws against monopolies in fields of business. All it does is give one Union disproportionate power to f with the market and gives them the potential to bring the country to its knees over relative inconsequentials.
Unions shouldn't be about monopolising and striking, they should be about securing fair conditions (and a legal safety net) for their workers and also assisting the management to run the business concerned efficiently.
Thing is most large Unions were created by mergers (case in point is Amicus, which was a merger of the AEEU and the MSF. Both of those were created by mergers themselves) and as most U.K Unions are vertical/industrial the negative effects of big mergers aren't as big a problem... if the same thing happend with craft unions, there'd be trouble.
Besides the most militant unions in the U.K have tended to be smaller ones (FBU, RMT, ASLEF etc) while both Simpson and Woodley (who were both painted by the media as hard left ultra militant nutters who'd strike over paperclips, when they were first elected) have behaved quite well (I'm actually slightly suprised at Woodley's moderation though. Maybe because he's associated with Longbridge).