A significant minority want to end this (basically to create car-free cities).
By rights the land is council owned, there's no reason it should be used for parking, and yeah, £300 a year is far under the market value of the land if you put even a camper van on it.
But personally I think it'd just end up ghettoising the inner city all over again. Families and anyone who wants to do anything with loads larger than a suitcase would move out to the suburbs or out of town entirely.
Some people probably consider that acceptable collateral damage. I think that places like London suffer from being too focused on single professional life as it is.
A lot of the discussion on cars seems to revolve around what "can" happen in some theoretical abstract, without considering whether people actually want that to happen. It seems very, central planning, for lack of a better term.