Rather than charge everyone £7 or more for a drop off, wouldn't it make more sense to charge the people abusing it an absurd amount? I'd much rather see a £25 fee after 90 seconds and an additional £125 fee after 5 minutes than £7 for 30 seconds.
It seems less about making things more efficient and more about just squeezing a little bit out of money out of everyone.
Recently parked in a Spanish airport carpark that worked similar to this.
First few minutes free, lower tariffs for 5-10 mins (or maybe fixed charge at those limits?), then like 1 euro per minute after that.