I think you need to look beyond just mere possession of a firearm.

The "barrier to entry" for a UK sourced active shooter attack is significant. A successful active shooter style attack, be it by a group or a lone-wolf, requires at least:

- some degree of training and familiarization with firearms;
- multiple firearms;
- ample supply of ammunition

Procuring these things in the UK significantly raises the risk of exposure for any active shooter plot.

I would also imagine the price of firearms on the UK black market is high - seeing as how the supply is low, and much of the demand would probably be from people in the narcotics trade who have the funds to pay a premium for weapons.

Obviously I have made assumptions here - but I believe they are fair ones, and the impression I get is that a homegrown active shooter style plot sourced from within the UK has significant risk of exposure and financial costs.