Can anyone offer advice on which airlines are actually flying connections from Bucharest to London? I've done a lot of hunting, but many posted flights aren't actually flying. This would be for a Saturday in August, one way. I'm not asking about covid controls--asking specifically for flight logistics. Hoping to avoid LCC. Thanks!

Schedules for August remains as a placeholder for most airlines so there is no guarantee that a flight that you book at this moment would operate.
TAROM ( direct ), Lufthansa, Austrian, Swiss, Air France, Turkish Airlines, etc... all fly both to London and to Bucharest ( at the moment ) and I would consider them safe. Although your safest bet would be Wizzair and Blue Air which are LCC's.

BA plans to resume Bucharest on 5 July 2021 and unless there are further travel bans, they would be most likely operating that route in August.

Also a reminder that, if the current guidelines stick when you travel, you would need to self-isolate upon arrival at the UK for 10 days ( 5 days + test out is possible ) as Romania is in the amber list of the UK. For reference, at this moment, the UK doesn't have any exception for vaccinated travelers.

Thanks, ISTFlyer! This is very helpful, esp regarding BA resumption of flights.

Yes, well aware of the UK isolation and testing requirement. The traveler in question has a home in the UK, so not a problem.

You're welcome