Earlier quoted context omitted.
Such actions are covered by the UK definition of terrorism: http://www.legislation.gov.uk/ukpga/2000/11/section/1
Out of interest, are you a lawyer? I'm really struggling to see how you can interpret the language of the page you linked to include non-violent activism. Could you tell me specifically which sections of that legislation you think apply here?
1b : the use or threat is designed to influence the government or an international governmental organisation
1c: the use or threat is made for the purpose of advancing a political, religious, racial or ideological cause
So XR is an ideological and/or political cause, intended to influence Governments, and this threat to fly drones over airports in and of itself risks the public. Assume a failure and/or crash of the drone, and all the bad "accidental" things which could arise.
One could certainly argue that the statute is "wrong", and that this sort of action should be treated as some other criminal offence. However I believe it is capable of being argued as terrorism under that definition.
Hence anyone so charged could be subject to pre-trial detention of up to 28 days, and then may have to convince a jury that they should engage in Nullification.
Part of the complaints about that Act when passed was specifically that it covered stuff which did not involve violence (2b - 2e). Note that the criteria in Section 1 (2) are logically OR'ed - only one of 2a through 2e has to be satisfied.
If you believe you may need advice from a Lawyer, I suggest you consult one.