Your car isn’t roadworthy, that is it won’t pass a safety certificate. Without a functioning horn, as such, it’s illegal for it to be on the road.
And it’s illegal to drive because it’s dangerous as you already recognise. Worse, in a smash where someone is seriously injured or killed, the police would be checking the vehicle for safety related defects which can make things unpleasan.
So, in a place you say is full of diskheads, I’d be fixing it without delay and without concern about button batteries issues. As is, a dealer must be able to sort it out what the cause is without too much cost.