>Why to encourage the revolt, knowing well that promised help with not come or will be withdrawn, leaving Vendéen, Chouan, Calabrian, Tyrolian alone to face repression?<
It didn't cost the British anything to encourage it, and in exchange they got unrest among the populace, forced the French to engage in repression and occupy troops, and make the French look bad. If a few Vendéens, Chouans, Calabrians, Tyrolians, or Spanish have to die, that's the breaks.