The payload must be several kilo then if they carry cameras used in industry. When they use it they will probably obtain a small amount of plastic explosive, a small live camera for relay via net streaming, mobile phone and detonator or such. A flying IED. Terrorist emphasis will be upon surprise, accuracy and proximity to a target upon detonation and a defenders initial inability to counter.
It will come. In fact it has probably already been used somewhere.
I've no doubt our counter-lot have already thought this all through (a la Olympics and the air defence installations).
Is it a police job to defend against though, or RAF? I'm just interested, and I'm sort of surprised the RAF in an age of constant cutbacks, don't try and plug themselves a bit more here (turf protection for jobs and careers etc).