A lot of people wondered about the wisdom of using military assets for this! See the thread on C17 for Tunisia.
Once the Govt decided (or, rather, once the 'special advisors' had decided) to use C17, Brize was the obvious place to put it because you don't have the same difficulties with airside access that you do at commercial airports, and the ground team is familiar with handling the aircraft.
I can also see the sense in using military personnel to offload the coffins as (sadly) it is a known process and there is full awareness of the hazards and techniques involved. I suspect a typical funeral director would have struggled with a heavy coffin and a C17 ramp. The alternative would have been a forklift or similar - hardly dignified.
Your question about thresholds is entirely valid, as this case shows why you should not make up policy on the hoof. Where do you start and stop? The last full ceremonial repat of a civilian was for Princess Diana, who wasn't really a civilian anyway. There will almost certainly be some people whose nearest and dearest came home as freight because theirs was a non-operational death in service and who will now be asking what makes the Tunisia casualties special. The answer to that question may be about political signalling, but it might equally be about failure to heed advice or to think through the consequences of the decision.
The trouble now is that a precedent has been set which will drive public expectation in any future event. And if you set a number (15?) you can imagine the pressure to match the Tunisia response if the actual number of dead turns out to be 1 under par. Someone is going to have to set some qualifying boundaries, such as armed terrorism + mass UK casualties abroad; better still would be for No 10 to declare it as a political message and a one-off event that will not be repeated.