The place of supply is where the customer belongs, which generally means where they have offices. The location of the event is irrelevant.
So, you charge UK VAT if you supply a UK company with no overseas offices (i.e. they belong in the UK only) at a UK or overseas event.
If you supply an overseas company with no UK offices (i.e. they belong outside the UK) for a UK or overseas event, you don’t charge VAT as they are outside the scope of VAT.
If you supply a UK or overseas company which has offices both in the UK and overseas, you determine the office most directly concerned with the stand. If it’s the overseas office, you don’t charge VAT as it’s outside the scope of VAT. If it’s the UK office, you do charge VAT.