I've had issues with some varities (like canik) being somewhat flame retardant. My guess is your tobacco may not be dried sufficiently though. The U.K. Is probably fairly humid in general - you're living on an island after all. Have you tried storing the tobacco in a controlled environment where you can get the humidty lower? Think possibly a humidor for a few weeks with a lower relative humidty, then see how they do maybe. Finer shreds may help but not if they are packed tight and still to wet.
I would think Chlorine is probably fairly volatile if you are watering with tap water, although others might disagree. I would assume most of the chlorine off gasses pretty quickly, but I don't have any scientific evidence to back that up - other than most chlorine will evaporate from tap water in about 24 hours- no idea how it might bind to other substances in the soil though. I rarely water my plants with tap water once they are established so it's not really been an issue for me. Dry tobacco burns, overcased moist tobacco doesn't burn without a flame held under it in my experience. Nothing much more frustrating than trying to keep a freshly rolled cigar burning in 90% humidity while the mosquitoes are biting and sweat is dripping off your forehead in 90(F) + temps... Wow, I just got myself looking forward to summer.
