There is actually a company doing this in the UK, water machines that you can subscribe to and fill up they're called Spotless Water, but my local aquatics shop is much closer.
Hi mate I'm also in the UK, I restarted my aero grow this week and while I'm waiting for an RO filter to come I bought 25 litres from my local aquatics shop for £3.50. I've done whole grows in the past getting the water from an aquatics shop, it's not a ridiculous price to pay.