They're illegal in the UK.There is a delivery driver for the opposite office block who turns up every morning at 10:30 with that exact horn. At first we found it amusing, now we just wonder why!
I'm guessing that it's more of a "your delivery has arrived" alert than the main horn of the vehicle - but obviously have no way to tell.They're illegal in the UK.
I believe there is a pre-date before which they are legal but I'm sure its 1970 something. Any car after that date can't have a musical horn in the UK.