According to the Motor Vehicle Construction and Use regs 1986 (Clause 37 if you want to look it up), "no motor vehicle shall be fitted with a bell, gong, siren or 2 tone horn". The horn must be "continuous and uniform and not strident". However, one of the exceptions is "if the purpose thereof is to prevent theft or attempted theft of the vehicle or its contents". Make of that what you will, but I think you might get away with it as an attack alarm only...not the normal car horn.
www.legislation.gov.uk/uksi/1986/1078/pdfs/uksi_19861078_en.pdf