They can, but typically they don't as far as my anecdotal experience tells me. And I would imagine a lot of those 7m are the sectors we hear about in the news, like aviation.
That's on the union, pick a better one. Get better reps.
Do you mind me asking waht sector you work in?
Business consulting. I used to work in (office roles) in manufacturing. I'm not in the UK but my union is one of the biggest, and very (scarily) efficient in many ways.
There are things that unions can't do - you have to check if they provide legal support, or discounts or whatever. It works out much cheaper than legal insurance for me.
Strikes only happen, IIRC, where a workplace is unionised and there are reps or in places (like where i am) where industry wide tarrifs apply. Kindergarten workers, for eg, are covered by my union. A Tarrif for wages is applied and agreed in this industry. When they weren't offered better conditions they went on strike. It was very effective. I wouldn't be able to strike because no tarrif, no union in the workplace (i am in a union because unusually for here we don't have a workers council)