Most people don't travel from that far frankly; I reckon at least 90% of people visiting live under a 2hr drive from the park. Leaving at 8am is hardly an unreasonable hour.
The trouble with closing later is a culture thing. British culture very much revolves around 'get in early, get out early' so people can do other things with their evenings. Convincing people to stay at a park later in the UK is difficult and, to date, the only thing that really works is Halloween / firework events. Outside of that, it's tricky to convince people to stay.
Is there a difference between cleaning the park before it's open as opposed to after its closed? Either way you're having to pay for staff to be there for the same amount of time.