The mindset of a man who's willing to set himself on fire to make a point — as one of the Glasgow terror-clowns seems to have done — but not to spend any effort at all on researching methods is a difficult one to understand. Even if these jokers were illiterate or had no internet access (seems unlikely, one of the suspects is apparently a doctor) they could have at least done a test. In my part of town, fun-loving teenagers burn out a car or two down by the canal every week or so: nobody would notice another one with some nails in it. Such a test would have told these idiots what every bomb-disposal operator and Hollywood effects guy already knows: that petrol, gas etc make for an excellent, photogenic fireball which you can normally be quite close to without ill effects.