Q406: When can I use my vehicle's horn?
Rule 112 if the Highway Code states:
'The horn. Use only while your vehicle is moving and you need to warn other road users of your presence. Never sound your horn aggressively. You must not use your horn
while stationary on the road
when driving in a built-up area between the hours of 11.30 pm and 7.00 am
except when another road user poses a danger.'