I presume that was rhetorical 
Yes. It would be illegal. Perfectly understandable, but technically illegal: the horn is there to tell other road users that you’re there. Absolutely nothing else.
And you can’t use elequent gestures instread, as you have both hands on the wheel, right?
In better news: I checked the exact wording of ‘The Road Vehicles (Construction and Use) Regulations 1986’, and it refers to any “horn, gong, bell or siren fitted to or carried on a vehicle”. I want a gong!
