I heard from a cyclist that motorists are legally prohibited from using their horns on the street to get people's attention or indeed for any reason other than to directly avert a collision.
From the Highway Code:
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.
Law CUR reg 99
Zebra and parallel crossings. As you approach a zebra crossing ...
- do not wave, flash your lights or use your horn to invite pedestrians across; this could be dangerous if another vehicle is approaching
- be patient, do not sound your horn or rev your engine as this can be intimidating ...
Animals. When passing animals, drive slowly. Give them plenty of room and be ready to stop. Do not scare animals by sounding your horn ...