Please provide evidence to back up your suggestions that withdrawing from a purely voluntary individual arrangement could be in any way construed as industrial action. If you'd been voluntarily helping out the company by working to 1730 (extra time unpaid) for a few months but then decided to start finishing at your contracted time of 1700 would this be construed as unofficial industrial action?
As evidence I can only point you to the relevant legislation of which there is a great deal. It is not specific about what is industrial action but there is no doubt that action short of a strike, what we used to call working to rule, is. What counts as action short of a strike is open to interpretation.
In your example if you give up for purely personal reasons then it isn't industrial action. However if you and your colleagues give up and tell NATS that you are doing so in support of a grievance then it almost certainly is. If you carry out that action without following the prescribed path then it's unofficial action. There's a whole range of options between those two points that only expert legal opinion can give advice on. It's a difficult call if for instance if you give up for personal reasons against a background of industrial unrest.
Unofficial action is not illegal but taking unofficial action removes a lot of the protection you enjoy as an employee. You lose most of your unfair dismissal protection and could be sued by customers for subsequent loss. If this ends up as a dispute then it's in every body's interests to act as directed by the union and ensure that any action is official. That is the point I was trying to make with my first post.
I am surprised at the number of people posting here who think that UK law is logical, fair or that you have rights. I'm afraid that isn't often the case. Often when legislation talks about the rights of the worker it's actually really a limit. For example your right to be consulted on industrial action is the part of the legislation that sets very specific ballot requirements which actually act to limit your ability to withdraw your labour.
I seem to be in a time warp. My last post at 17:24 is shown as 09:15