You'd have almost certainly been taken to court, and it would not have been too surprising if you were at least accused of treason.

Blackout restrictions were incredibly tight, and ruthlessly enforced. This first hand story tells of a young woman being fined the equivalent of 15p for having a lamp on during the blackout, the light of which could be seen from outside, and then taken to court for 'wasting electricity'. Often the fine was five shillings or so, and that was simply for having a crack in the curtains.

In could, in theory, depend partly where you were. It was very common for the Home Guard to light decoy fires in empty fields in order to draw enemy bombs away from populated areas. One supposes, if you were to light a bonfire in an empty field on the same night as the decoy fires, and scarper before the Guard arrived, you could perhaps get away with it.

However, if you were to light a bonfire close to a populated area, especially on a night when bombings were taking place, it would be highly probably that you would be accused of trying to draw the enemy bombers to that flame. Setting up a sort of 'decoy' decoy, if you will.

The main alternative was to instead hold it indoors:

"Five days later we had Guy Fawkes’ Night — with a difference! We held it around the kitchen table — an INDOOR BONFIRE NIGHT but with no bonfire, indoor fireworks only. They came in a small box and were all made of lightweight papers that floated up to the ceiling, by which time the burning had ceased and some were lit on a plate and formed themselves into burning, wriggling snake like shapes — what fun it was! [ Source ]"