Tap water or bottled water?

Bottled water is indeed a solution, but not one that is always the best. For me personally, environmental reasons are important to me. Buying bottled water is pretty wasteful.

There are some other options as well if your tap water is not of the desired quality. For example, you could look into water filters like the jugs from Brita or Peak Water. You can also see what water your local cafe uses and maybe you can get some water from them if they treat their water themselves. Obviously, they might not do that for you for free.

The water in London is hard enough to nail to the wall in sheets;)
High calcium [& magnesium] content is the very definition of 'hard' water.
It is so bad here it actually forms a strange almost crystalline-looking scum on top of tea or coffee in some of the worst-affected areas.

Check this map from https://www.aquacure.co.uk/knowledge-base/uk-hard-water-map for a general guide to the UK's water hardness.
Light blue is soft, dark blue is hard water.

I grew up in Yorkshire with some of the softest water in the country [which is precisely why in older times it was the centre of the woollen industry - things wash better in soft water, using half the soap], but spent the past 30 years in London. Until I moved here I thought water filters like Brita* were just some affectation for rich people. They're not, they're to stop London water tasting so bad, get rid of the weird scum on your cuppa & prevent your kettle looking like Mother Shipton's cave after two weeks ;)

_{Image from Yorkshire.com - Mother Shipton's Cave - a popular tourist attraction known for the calcification of objects hung in the dripping water.}

With a jug filter, I need to descale my kettle every year or so & my coffee machine never, as it has its own similar filtration built-in.

Bottled water has too many variables. It doesn't quickly indicate hardness, you have to read the small print & extract the relevant information. It's also ridiculously expensive compared to filtration. Drinking bottled water comes from the days before all domestic water was potable & remains only as the above-mentioned affectation. There's nothing vaguely 'unhygienic' about any British water [nor the entire EU for that matter].

So, buy a filter jug [or if you are rich, a tap filtration system] & enjoy your coffee.

*Other makes are available, this is just the most common in the UK, available in every supermarket.

We get our tap water from local sources, such as rivers and lakes. This water may contain bacteria, industrial pollutants, agricultural runoff, and heavy metals and minerals leached from the soil. In order to make the water safe to drink, a treatment plant must filter out particulates, disinfect the water to kill microorganisms, and remove any minerals or other chemicals that exceed the standards set by EPA. And yes Generally speaking, thanks to municipal water treatment facilities, the water that comes out of the faucets in our homes is safe to drink. So simply bottled water is best

I would use distilled water. It gets out more impurities.

Reverse osmosis water is the best for making coffee and drinking. It tastes way better than my tap water.

Bottled water is not typically safer than tap water.In fact more than haft of all bottled water comes from the tap.Buying Bottled water is like pouring money down the drain.

I didn't know this till I just read it:

Why Your Bottled Water Contains Four Different Ingredients

Many of the ingredients that are added to bottled water occur naturally in tap water and in our daily diets.

If you like to drink your tap water then I would use it for anything, including making coffe. Where I live in Portland OR, our water comes from the Bull Run Watershed and it's some of the best water I've ever drank. But when I lived in Santa Cruz CA, that water was pretty bad and I would always use "bottled" water for everything except washing.