Convention-goers can wrongly assume that some laws do not matter at a convention.
They might take some recreational drugs.
They might smoke indoors.
As the organiser of the event you could be in trouble if you are seen to endorse these activities. Make it clear that you will not tolerate them and be prepared to enforce them if necessary
You are liable for a £2500 fine if anyone is caught smoking in a public indoor space at your event.
As a worst case scenario your event could be closed down if police discover illegal activities taking place.
"The police have the power to close down premises or outdoor events on the grounds of disorder, the likelihood of disorder or because of public nuisance caused by noise emanating from the premises."
You may want to consider in advance your policy on throwing people out whose behaviour you seriously believe to be a risk to the convention.