In reply to apache:
Was there last (Tues 26th I think), it took my wife and daughter an hour to check in and then over an hour to get through security. They could not check in online as they needed to show proof of PCR test but this might be different for you, they were going to Japan. I've never seen the security queue so long, I think it took them about a hour and a half to clear it but they made their flight.
I'd err on the side of caution and get their really early though it does depend on what time they open the check-in desks (if you need them).
We live near to Heathrow and tried to drive but the local roads were really busy so we ended up taking the Heathrow Connect from a local station and I'm glad we did. If we'd been stuck in any traffic near Heathrow they probably wouldn't have made the flight.
Good luck