Not been own-brand (neither expensive “mineral” nor own brand cheap fizzy tap water) fizzy water in local Morrisons for months! Was not in any old-brand supermarkets for a couple of months at least (Lidl still had some) but Sainsburies got fizzy tap water now and Tesco have own brand fizzy mineral water. BUT!!! sure the Tesco mineral water is less fizzy than it was! That is to say it used to be impressively fizzy.
The thing is, this is a several years old problem, I researched it and it is to do with carbon footprint and energy costs and shutting down production of compressed CO2 in the UK. Our leaders are severely substandard it seems to me, except in the fields of bullshit and cover-up at which they excel!