Instructions:Just tried it and worked for London.
They seem to be the cheapest (for entire day) for Westminster even before the discount code. Especially for same day parking.
Also they seem to quote all day rates (i.e. same no matter what times you put in), so worth booking for entire day just in case. As if you stay beyond the booked time the standard car park rates will apply.
NB: Hopefully HUKDealers don't fill up all the spaces next time I need to book :)