Not sure as £50 per 2.5 hour session is enough....
Ingredients are going to be say £10 (atleast?)
Time sourcing and prepping ingredients, so lets say you spend 1 hour doing this.
Plus 1 hour travel to and from a location (if you do it in their house)
A 2.5 hour session, which realistically is going to be about 3 hours of your time to unpack stuff, set it out then clear up.
So in total that is 5 hours of your time for £40 = £8p.h gross - not a huge wage.
Plus petrol/wear and tear on car.
If they come to you that would take out an hour of your time bumping up hourly rate to £10p.h.
Think you need to charge per head as notasize said.
Other thoughts...
Are you demonstrating or do you want people to actually do the cooking? Will they get to eat it? Could you offer a kind of 'learn to cook and party' type of thing, say 3 girlfriends learning to cook but then get to eat it and drink wine at the same time, so it's a girls night out aswell?
Think it is certainly achievable but as you say lots to think about!