It's perfectly OK. Here is why.
So the chargers near The Bridge pub are 7kWh chargers.
They charge using AC rather than DC and will charge at the same speed I can charge at from home. These chargers add around 33 miles of range each hour of charge for my car.
Therefore it could feasibly have cars sat on them for 1-7hrs depending on the state of charge when they arrived and no other restrictions in place for charging.
Of course it is poor etiquette to leave a car on there once it is fully charged and you should unplug and move if you have enough charge, or fully charged.
The chargers that allow the 20-40 min charging times are ultra rapid chargers and operate on DC. Rapid chargers are usually 43-50kWh and Ultra Rapid are 100kWh-350kWh.
Some cars can't benefit from the full 350kWh but most will take 100kWh as a minimum. I las went from around 25% to 80% in about 18mins using 250kWh chargers.
If you are not in a rush to charge, using these slower chargers are actually better.
If you know you are going to be 3hrs, it's better to take up a slower charger than a rapid one and leave a space blocked as your charge is done in 20 mins. Just common sense really.