There’s so many variables to this. iF your flight inbound is early or on time and you are through checked I’d see the lay of the land on the day.

immigration could be 5 mins or could be 45 mins or longer. . If travelling to London by Heathrow Express I’d say you need to be heading back to the airport 2-3 hours before. Some say that’s far too much time but I’m a pragmatist and factor in the ‘what if’. Sadly if you’ve allowed 2hrs and there’s a signalling fault or passenger incident then you’ve got virtually no time to find an alternative back to Heathrow to meet conformance.

5 hours is below my comfort zone . Windsor May be ‘safer’. But whilst people are travelling througH unhindered others are having 90 mins wait for check in, stupid times for security, The First Wing closing at 3pm because it’s short staffed.

At a push I’d say pop to The Sofitel for Afternoon tea if you don’t have CCR access.