Not wanting to make it worse than it is - but this is from the gov.uk site
Example 1
You leave a non-exempt country on Monday morning and arrive in England on Monday afternoon. You must self-isolate: on the day that you arrive back for 14 full days AFTER that
Example 2
You leave a non-exempt country on Monday afternoon and arrive back in England on Tuesday morning. You must self-isolate on arrival and for 14 full days, including the Tuesday.
I have to say I believed the rules were as you said Spurs. So legged it out on after 14 days in the UK (landing at 18:00 fro ALC) but looks like I should have stayed in one more day
"led by the Science"

Been through this a couple of weeks ago current rules are if you are staying in the uk after arriving for 14 days or more you must isolate for 14 days. If you are staying less than 14 days then you must isolate for the entire time . Your isolation period starts from when you step on Uk soil so if you arrive at 2355 that is still day 1 even though you have only isolated for 5 mins it’s a day done. It says nothing about time of arrival just that the day you arrive is day one and you must travel directly to isolation place.
Interestingly and I checked with helpline anyone in the house where you are isolating does not have to isolate unless they show symptoms and can go out and visit others etc as per normal rules etc. But no one else can visit while you are isolation.
Confused so was I