They had enough stones to set themselves on fire in the attempt. Given the erratic behaviour of both vehicles prior to 'detonation' my guess would be that they tried to let the fumes build, showing the understand how petrol explodes, but not how humans react to concentrated petrol fumes! This isn't Al Qaeda rejects. This is Al Qaeda scared by past police monitoring into not using traditional explosives/explosives making materials.
Sand, I suppose it has ocurred to that firstly we do not have the death penalty in the UK, and in any event that death would serve no purpose. It would certainly not make them any less happy, and would deprive us of the opportunity to let them stew and work out they'd made a ghastly mistake.