I’m no expert on parking restrictions, (but I know a man who is and he will doubtless reply shortly), although it may be that the permitted variations of TSRGD 2016 S4-3-2 might give you what you want.

For example the circular symbol from S4-3-2 accompanied by the following text across however many lines:

No stopping except for buses 6am – midnight and taxis midnight – 6am

Quite how, or if, this would be enforced, especially around 6am, is another matter altogether.