Why do you expect they will be struggling staff wise? I doubt that there will be more problems than normal
As others have said, teachers normally struggle in when they're ill because setting cover takes ages and we all know the kids won't do it anyway which means we'll lose precious time which is especially bad if they're an exam year.
The staff room in the autumn term is always a plague zone in normal times. Plenty of coughs/temperatures etc but normally we'd drink lemsip / dose up on paracetamol and come in.
Now, everytime we have a cough or temperature or loss of smell or taste (I get this every time I have a cold), we will have to isolate and take a test. Even if it's negative the guidance says we should stay off if we're still suffering covid symptoms. Probably because (a) the tests aren't 100% reliable and (b) these symptoms indicate a respiratory infection of some sort and if someone gets this at the same time as covid the prognosis is much worse.
It's not just the staff though. If the close contacts of a positive test need to isolate that could knockout around 20-40 kids for 2 weeks. One asymptomatic kid could spread it to that many of their peers and no-one will notice until some of the others develop symptoms and then each of those will knockout another 20-40 kids. That much absence is going to have a huge effect on continuity of education. Unless the whole year group is isolating, staff won't have time to provide any sort of quality remote provision so it'll be oak academy / bitesize / online textbook.
I wouldn't worry about strikes by the way, despite media representation the unions don't hold any actual power and the last few months have made it very clear that we (teachers) don't have the respect or backing of the general public.