The PH cars in my city all have stickers on their back doors which say "Illegal if not pre-booked" which I guess pretty much answers your question.
The Private Hire Contract is just that - a contract involving pre-booking. Only a Hackney Carriage license holder is allowed to pick up a hailing passenger. In fact, the literal law states that HC cabs can only pick up when stationary, but all police forces turn a blind eye to this because in major cities like London, the traffic would gridlock instantly if the law was applied.