Is this for an espresso/filter machine?
I always just used tap when I lived in London and had a V60. It’s hard as hell but it tasted good to me and nothing other than the kettle at risk of scale. Surely nobody is using RO water for cooking?!
If it’s for a machine, I’d second the above, just bottled. Waitrose Lockhills or Volvic.
Lockhills is £1.80 for 8 litres. 45p a bottle.
So almost 1000 bottles for the price of an Osmio…