This summer, honestly, it's not worth the trouble and expense of planning to fly here from the US to see theatre, even if/when quarantine restrictions are lifted (at the moment any arrival from a country on the amber list who is not fully vaccinated *via the UK's vaccination programme* must quarantine for ten days on arrival). The situation here is such that a significant number of performances are being cancelled at short notice; it would be a pity to travel all that way for a trip based around theatre performances and have half the shows you planned to see cancel, and that's certainly a plausible scenario. And I believe the CDC just issued a Do Not Travel advisory for the UK, based on our sharply-rising case numbers.
I do think things here will calm down, despite the best efforts of the sociopathic blond idiot in Downing Street, but they aren't going to calm down quickly enough to make a summer trip to the UK an easy proposition for overseas visitors. And there isn't the range of theatre on offer that there would be when things are running normally. For this summer, in your position I think I'd wait until things here are a bit more stable.