To do the MUR is all any ATCO is required to do, anything else is done as 'goodwill',
I'm no manager nor am I a lawyer specialising in industrial law but since the Thatcherite anti union legislation your rights to withdraw your goodwill have been severely curtailed. You can withdraw those things at any time as long as you don't do so in the context of industrial action. And that's the problem because unless you tell NATS why you are withdawing that goodwill it doesn't achieve much and as soon as you tell them it could be considered industrial action.
This is a very complex area of the law and you need to be very careful. If you don't believe me go off and research it yourself. DIRECT.GOV.UK is often a useful start.