Minnie, a pp has already said how they manage it in their school. The HT takes half the class. The focus is then on all the children from the year who haven't had the opportunity.
How do you decide who goes?
Well, when I was a child you got a free pass if you could swim 25m. The National Curriculum also says 'should be able to swim 25m', so I'd have thought proof of that ability (certificate, or badge for most, demonstration at first lesson if that isn't available for those who can't produce them).
Some people may not think their kids benefit from school swimming lessons, but even if they can swim it doesn't harm them to get some extra practice
No, tbf, most school swimming lessons are a complete waste of time for competent swimmers. (I'm agreeing with you are a necessity for those children who haven't had the opportunity though, so that's why I think the whole system would benefit from being looked at).
OP, what is the purpose of your survey? All children who are able should learn to swim in primary school - it is a basic skill and could save their life. I think the point is that school swimming lessons don't usually improve the swimming of children who are already competent swimmers. It does seem sensible, to do that same as you do with other subjects (where the staffing is possible, and I realise it isn't always), that is, give the extra input in a smaller group of those who need support to get up to a basic level.