As you are in a shared home situation, @Barns I think it’s always best to talk in person to guests about checkout time before check out day, so there is no way guests can claim ignorance.
I’ve noticed many guests don’t have their Airbnb profile set to receive notifications of messages, so they may not well see a message.(of course they should know what the check out time is, but as we all know guests don’t always read).
In your situation i would have knocked on their door at 11.40 to remind them they are way past their check in time.
I think agreeing early check ins is always a risk @Barns when you have a same day turnaround so personally i never accept them unless I know the previous guest has an early flight to catch.
I would knock off one star I think, rather than two and mention it in the review.
And you live in one of the best cities in the UK with loads of lovely places to get a coffee and something to eat, so its not like your early arrival guests can’t keep themselves entertained for an hour.