In the past I’ve checked MOT requirements and the Construction and Use Regulations and found no reference to loudness. Seems to me if the horn is electrically powered, emitting a constant tone and is loud enough to be heard it complies.
Then again, what do I know? Someone will be along shortly with a source and reference to the contrary.
The Stebel is a claimed 140dB, but it really isn’t that anti-social. MOT test has no upper limit on loudness, just a lower limit. Do wire it to battery via a relay though, else it’ll be really quiet. The Stebel Nautilus can be split in half too, mine came with a pipe to link the two halves is required. Anyway, here’s the Stebel kit on ebay £32 delivered with relay, wires and crimps. Just need to fashion yourself a bracket.
Remember that the Stebel Magnum you can probably just do a straight swap, the Nautilus and similar you’d may have to put in a relay and new wiring as they do pull more amps than the standard wiring can cope with.