If you work for NATS then you will have to declare your secondary employment. Some years ago the rules were changed to ensure that ATCOs did not work in areas where their job as an ATCO could be compromised. That was specifically aimed at those who were doing ATC type jobs on days off or leave. Having made enquiries at the time the management view was that the ban should include any CAA licensed activity.
Don't know what the latest interpretation is but obviously those who were already flying were not stopped. Might be worth a quick check with your local manager