You mustn't use your vehicle's horn between 11.30 pm and 7 am in a built-up area or when you're stationary, unless a moving vehicle poses a danger. Its function is to alert other road users to your presence.
Did you know that the current waiting time at the Isleworth (London) - relocating to Isleworth (Fleming Way) on the 14th May 2018 driving test centre is 3 weeks?