however most schools only provide this in years 5 and 6
Lets stop at this point. Let's ask the fundamental question, why do they only offer it in years 5 and 6?
The number one reason comes down to shortages of local swimming facilities and swimming teachers.
This is compounded by the cost of the lessons that has to be covered by parents or schools in someway. This is the cost of the hire of facilities, paying the teacher and the cost of transport.
You also need to factor in the number of staff you have to allocate to getting the kids too and from swimming lessons - younger children require a higher staff ratio off the premises than year 5 and 6. So where are those staff coming from?
There simply isn't the budget for it. And there isn't the facilities for it - they'd have to build more.
Local swimming pools have closed down at a staggering rate in recent years.
In terms of how much of a priority this is compared with other funding aimed at improving outcomes.
I'd also be curious as to whether findings that:
children who learn to swim earlier reach major cognitive development milestones including speech, literacy, numeracy and visual motor skills much earlier than non swimmers
is weighted for socioeconomic backgrounds or whether parents who get their children to learn to swim are simply more involved with their children which is the single biggest factor in improved outcomes - thus it being a correlation rather than a causation.
Good luck to you OP. I think it's a noble aim however, you need to stop the closure of local leisure facilities and get councils to fund new public facilities for all as a starting point before you could ever have this as a realistic goal. Those facilities would need to be financially viable to be sustainable. That's really not the case at present. Lessons for kids would be a secondary goal to achieving that.