I thought it was 10pm, but you need to check your local byelaws. Round here it's not taxis, it's the youngsters calling round for each other. Ten to midnight is out of order!
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