The Les Mis that (re)opens in May in London is a "staged concert" with a socially-distanced auditorium in which, yes, a significant number of seats were never on sale. The full production is supposed to return in late September.
Based on the NHS's vaccination schedule, late summer/early autumn seems feasible for shows to open at full capacity (in theory, every adult should have been offered at least a first vaccine dose by the end of July). The government's current date, if they keep to the schedule they laid out, for ending social distancing restrictions is June 21st, and while case numbers right now are quite low in most places in the UK that seems a little optimistic to me.
For myself, I'm not comfortable back in a theatre until after I've had both vaccine doses (first one next week, which puts the second one somewhere around the third week in July) - but I have a ticket booked for Hairspray at the Coliseum in September and for Hamilton for my birthday in November. I'm sure I'll wear a mask, whether or not they'll be required, and I'd prefer some kind of vaccine passport system to be in place.