There is always a threat to report them to trading standards for unfair practices,but they may insist that their request is reasonable because of insurance guarantees or whatever
If you were registered with Napit you would be able to use them to argue the case, usually they are successfull
If you are with Elecsa then they may not be able to help unless you are pir registered with them as well
If you are not registered, but are qualified,hold public indemnity insurance, then I would go the trading standards route or enlist the assistance of my local MP
Telling him how this company are discriminating against the local workforce in favour of unskilled enterprises
Who are these insurance people ? for what its worth the creeps would have a letter off myself explaining the error of their ways