"Why dont we call it OVERTIME like every other company?"
"Overtime" in NATS is different to an AVAA. If you work overtime you are subject to the Green Book T & Cs. If you work an AVAA you are subjuct to the T & Cs of the AVAA agreement in the last pay round. Call it enhanced overtime if you like.
"We could on the other hand stand up for ourselves and refuse."
Refuse to do what? If we are asked to work in a manner that is compliant with the Staff Handbook and SCATCOH we can't refuse.
Our management don't want a fight or bad feeling, they'll offer cash at the next pay round to accept a deal that they could probably force on us if we say no.
They may want to get us to accept changes to our T & Cs that they couldn't force upon us, that'll be another wedge of cash.
As HD has pointed out there will always be an ATCO willing to take the extra cash. Actually, it's probably a majority of ATCOs now, judging by the last pay round.