I use frozen 2L bottles of water placed around the FV. Two of them is perfectly acceptable to maintain lager temperature around 11-13C here in the UK with 25C ambient.
When the beer hits FG, I place four 2L bottles (frozen) around the FV and have got the FV down to 8C at the time of writing...
Agreed - two weeks is a LONG time for any ale of modest gravity to be fermenting. I find most of my beers are at FG within 4-5 days. If I’m dry hopping, I’ll add them around this time. If not, I’ll add finings and chill for a few days before bottling at day 10-14. I don’t want it hanging round...
This is their cheap stuff that does not list alkalinity/bicarbonates, but does list dry residue at 219ppm... https://www.tesco.com/groceries/en-GB/products/301456121
I know they changed their smart-price brand water recently, as above, but the Ashbeck stuff that comes in 5L bottles is still low in calcium and alkalinity. Their website might just have the figures from the cheaper bottles. I’d suggest going to the store and looking on the bottle.
If you’ve got Tesco Ashbeck, you won’t need to reduce alkalinity- it’s already 22ppm, so perfect for light beers.
You can add some gypsum if you want to increase calcium and sulphates. Depends on the style of beer you’re brewing.
In the absence of big supermarket chains, you probably are best off just using acids to reduce alkalinity. I’d also get a water report so you know what your mineral content looks like.