To me it ties in to the whole growing up. Sometimes, you have to just leave childish things behind, and they start to fade. This includes Neverland.
Growing up entails certain sacrifices, and in Peter's case, it was Neverland. He'd grown up and no longer had a place there.
So many stories, so little time.

Well he wasn't living in London so he couldn't go back that easily, but yes he could occasionally visit. Since fairy dust runs out after a while though he would need a way to contact Tinkerbell too. Yes it could be done, but he would have had to plan it out.
I think the reason they did it like that was just to create an ending that represented a fully closed door on the story. Like the other poster said, he likely would go back to his adult life but would ultimately be a better person. Implying he'd never return likely was just a plot device to show that this was the end of his story beyond being a normal husband and father.

Would he have to be in London every time he went there?
I think once he gets back in to the swing of things (work, family, etc) he'll grow up again but as a better adult and thus wouldnt care to go back. He tood the Boys he'd always remember them but now hes a dad and he cant ever come back.