Hi @Simon2362 - This expectation is a HUGE difference between guests in the UK & USA/Canada/Australia - maybe the whole rest of the world! I've read many posts here from American hosts, where the view seems to be - you've rented the room, you should have access at all times, because you've paid for it, & under no circumstances should hosts (home owners) enter the room during the period when the guest has checked in. (Airbnb seem to view this as a serious offence, for which hosts can be de-listed.)
I absolutely agree with your observation that in UK B&Bs & Youth Hostels you are expected to leave in the morning, even as early as 10am & not return till 5pm. That's always been the norm!
But I think most guests would be shocked if I insisted on this....so I go with the flow. I've had an American guest stay in all day to catch up on internet stuff during a several month tour, I've hosted a British chap who walked in the morning & worked on his PhD thesis in the afternoon, and vice versa, I've hosted a Czech couple who did 2 demanding mountain walks on Good Friday & Easter Saturday, then returned at Midday on Easter Sunday for an afternoon sleep! I also host workers, some of whom are on night shift, so sleep during the day.
Regarding the key, I had the same dilemma as you. But I DO issue keys, in order to be accommodating. (A combination lock would not be good for me cos I let out more than one room & also have regular lodgers, so can't lock them out by regular combination changes!) I've recently added a midnight curfew to my house rules, cos I can't relax when folk are still out & coming back at 2.00am! I would advise: IF you wish to restrict daytime access, or a key - make it clear in your house rules and state clearly at the TOP of your listing - as guests notoriously don't read very far down!
Good luck!