A reason for this was not given, and in one case at least, the person at the switchboard could not (or would not!) answer as to whether this was so that the the activists could be contacted again, or for some other reason. In this insance, too, when asked about the legality of logging phone numbers without the permission of the people involved and for unstated reasons, the switchboard operator just hung up.
I urge those who are interested to look into the legal issues surrounding this, especially if these details are passed onto a third party.
There is no law against registering a complaint. And no law against lots of people complaining at the same time. Companies cannot allow all those who complaing about a particular thing be treated as a (borderline) criminal nuisance and keep their contact details for mysterious reasons.
In bureacratic terms it seems just as ludicrous as if all those who complained about airplane leg room were singled out, then herded together, and their phone numbers all given to chief executive to play with. In terms of policy or law it makes no real sense, and so must be seen as a deliberate way to intimidate protesters.
BMI is a commercial airliner, it has no obligation to carry out deportation flights and the public have every legal right (nevermind freedom) to express their disagreement with the choices the company has made.