Audible warning instruments 37. (1) (a) Subject to sub-paragraph (b), every motor vehicle which has a maximum speed of more than 20 mph shall be fitted with a horn, not being a reversing alarm or a two-tone horn. (b) Sub-paragraph (a) shall not apply to an agricultural motor vehicle, unless it is being driven at more than 20 mph. (2) Subject to paragraph (6), the sound emitted by any horn, other than a reversing alarm or a two-tone horn, fitted to a wheeled vehicle first used on or after 1st August 1973 shall be continuous and uniform and not strident.
Fitted one to our VW T25 campervan some years ago, the MoT guy couldn't hold his pen still due to laughing uncontrollably at the Klaxon. It passed several MoT's without any issues and-was-a-real-hoot