Water Hardness

As I have found out here that water hardness is important when choosing your fish.
I know this might be a silly question but the UK must have 100s of different water hardness parameters due to water source and water companys so no real UK average. So my question is me being in Leicester for instance and all my local Aquatic stores in Leicester we must have same water parameters as using same water source and Company.
So how do the shops sell all types of fish that are for hard water and soft water etc. Most shops I have been in are just plain tanks mostly without substrates etc for cost reasons so how do they get there water to suit the fish as as I've heard it is not that easy to change the hardness parameters.
I have even seen tanks with mix of fish in that have fish in suitable for hard water and fish for soft ?.
Sorry for long winded question but trying to learn as much as I can about this hobby to give my fish the best care

Just to add that the best way to check water hardness is using your water company website - there should be an area to enter your postcode and find out about your water quality - if you enter the postcode of your fish shop you can see if you are supplied with similar water (this is not always the case as water companies do not blend water to make their product consistent - rather it depends which source your home is most commonly fed by). Also worth nothing that my water softened a couple of years ago after an incident in the area where the water source I’m fed by was permanently changed.

@Johnno2009's already helpfully told us his water hardness here https://forums.thinkfish.co.uk/say-a-big-hi!/just-to-say-hi/msg49974/#msg49974, folks.   

Its 'good' in that it's neither very soft nor very hard so it doesn't exclude lots of fish as possibilities as such, in the way that my own extreme water would.