Addison Lee owner drivers scheme, you can probably log on and off as you please
OTS sends you e-mails, requesting quotes, which you can reject, or if you reply, they'll get back to you if they accept your quote.
Crawfords and Brunel probably want you to work fixed shifts.
Creative Cars probably ask you when you're available to do jobs for them, they have lots of tv location work so usually need a number of cars in one go after filming.
If you know the doormen, sometimes they remember you if you drop off at the same hotel regularly.
However, they often got their own contacts anyway, as well as black taxi drivers, or they've already got a contract in place with a PH firm.
Obviously if you go for it, you'd probably need an operator's licence, and maybe a facility to accept card payments.