Heathrow UK - Shanghai - Hong Kong - Heathrow - minimum time requirement in Hong Kong?

If I fly from the UK to Shanghai for a few days and then have a confirmed onward ticket to Hong Kong, is there a minimum time requirement in Hong Kong before returning to the UK? In other words, can i transfer onto a flight back to the UK on the same day I arrive in Hong Kong? Or do i have to spend a night in Hong Kong? Also do i have to provide bookings of my hotel stay in Shanghai when I arrive at immigration in Shanghai, or can I sort out my hotels when I am there.? I have two hotels I am interested in staying at, but have not yet made the reservation, and would prefer the flexibility of booking hotels once I have arrived. Thank you in advance.
Thank you Kevin for your reply above.