Re: Not just London
"There are a number of companies that offer phone to pay for parking services up and down the country."
And all the ones I've seen have been shockingly poor. Complicated, user unfriendly crap requiring you to arse around "registering" a load of irrelevant details, plus half a page of card details etc via a full blown web page designed such that it is only useable on a 27" desktop monitor. This obviously requires a data connection, something often not very reliable in a multi-storey car park, so it is slow and tedious to do, even on a top flight smartphone. As for the touch tone systems - words fail me, even impolite ones. The sort of customer experience that makes you wonder who they let into IT these days.
Now, I don't even bother with anything that says "pay by phone", because I'm confident that the developers will have produced a pig of a system that is slow, unduly complex and works unreliably. And the pity is, it must be possible to implement this by sending a premium rate text with your registration number with a car park code, or something similarly simple and user friendly.