Hi Ron
Halon should be a no no for you. It is being phased out as non CFC/ozone freindly or something like that. Also if you have a cockpit spray nossle, you may find yourself a little short of air to breathe as you try and get out.
Go AFFF as it is the stuff now accepted in all motorsport, or at least according to our rule books here!
If anyone is installing a system for use in UK they need to be aware of competition requirements (if that is where you intend playing with your car). Putting the trigger handle/switch in the cut out behind the door does not technically comply, although a lot of guys still have the pull handles there. The specified position for the extrernal pulls or switch is at the base of the windscreen on the drivers side. This is universal so all marshals know where to go irrespective of car, when you are crashed and unconscious....and on fire!
Electrical triggers are easy to install here but cable pulls are a bit more tricky. I run a mechanical cable trigger down to the front hinge bar and then back up inside the wheel arch. Works a treat but you need to ensure it does not corrode up and seize, so requires annual maintenace. I don't think readers here will have been introduced to my oily rag but it is a magical thing!
Positioning of the actual bottle should be inside the wheel base so it doesn't get wiped off in a crash. In the engine compartment is the easiest position but consider if there is an engine fire and the bottle pops before you trigger it. Also cable runs can become very long if you go to the front of the car.
Other positions people use is under the drivers legs (arguments for and against exist here!) or under passengers legs (no issues at all there!) or under dashboard where cabin heater etc go. Some even put it under front nostril section where GTD have the brake servos but this is potentially considered outside of the wheelbase.
Hope this helps
Oh yes, fires do happen. Once at Goodwood Tony Haliwell came into the pits with 4 feet of flames coming out the back of his 40. Marshalls quickly put it out but covered the car in the powdery stuff. Washed this off quickly as it can damage paint work (as if the fire didn't!) and then he had a look for the cause of the fire. It was a poorly made exhaust system that had blown its stuffing (even good systems blow their stuffing, so be warned) and had not been repacked. The resonance had cracked the steel casing of the silencer and blown super hot gases onto his number plate which had ignited. Then the bodywork and wiring loom caught fire. £500 later all back to normal. Got a photo somewhere of the mess. Personally I have cracked two silencers from this same problem so now keep a close eye on repacking them.
And there was the time Dave Parker was taking Jan his wife round showing just how good slicks really were. Came back into the pits and told Jan to get out of the car. She dilly dallied about a bit and Dave repeated his request and Jan told him she was getting out, whats the hurry. "We're on fire love!" Dave said, "F****** H***" said Jan and was gone!
Malcolm