A lot of fires are started at battery or main power cables. If your extinguisher is in the boot and the power is killed your boot won't open.
Also, an I correct in saying a 197 boot can't be opened if car is running?
Bolt it to the floor in the passenger footwell, half tucked under front of the seat. Use big washers or flat plate under the floor so the bolts can't pull through. Get a good quality, good size motorsport extinguisher and
It will have decent clamps. I'll get some pics up tomorrow.