CPAP machine in London and Paris

You can always just use tap water and clean off the scaling you get home.

The advice to use distilled water is to prevent a build-up of mineral deposits in the machine.

https://www.cpap.com/cpap-faq/Humidifiers#why-do-i-need-to-use-distilled-water-with-in-my-humidifier-chamber

It depends whereabouts in the UK you are visiting. In the south and east, water is predominately hard (high mineral deposits); north and west is predominately soft (low mineral deposits). See the map on the website below:
https://www.kinetico.co.uk/hard-and-soft-water-explained/hard-and-soft-water-uk

In the UK distilled water is not readily available so just use bottled water if you're in a hard water area.

For what it's worth, I stopped taking the humidifier on trips, even here in the U.S. It made for lighter packing and I can get by without it for a week or two and it was sometimes a hassle finding distilled water. Pretty sure you could safely use bottled water for a time. The distilled water is to prevent mineral deposits over the long term.

I have found large bottles of distilled water in actual "Target" stores, and in similar big-box places in Paris and provincial France. I would encourage you to look in a big supermarket in London, if not with drinking water then with laundry supplies or baby supplies. (That's where I've found it in Belgium, haven't looked in London.)

I might mention that I carry a water boiler pot for my sinus rinse, so you could settle for boiled and cooled water if you don't want to hunt down distilled.

Amazon sell it. Next day deliver it to your hotel when you get here. Easy.

there's no point in boiling and cooling water for this purpose as it does not remove the minerals which are the reason for using distilled. .

You can't find distilled water in London. Hard to believe but we were there two weeks and never found any. We were told car repair shops might sell it. We used bottled water.

I just use bottled water and clean out the reservoir when I get home

Deionised water in the UK is not that difficult to find. Distilled is a more specialist item

In Paris we found distilled or demineralized water at a Monoprix. There are two sectioons in the store where it might be. The clerks argued about it. Sometimes it is with cleaning supplies and you want it without minerals. You can tell on the label and the French word is close to American spelling. You want the kind of water that is used for an iron. Do not buy the demineralized water at a pharmacy. It comes in very small and expensive vials or they will not have heard of it. If you are checking your luggage, fill a 3 oz bottle of it at home.

I'm with Tim. I've used tap water in both cities the past 4 years for 2-3 week visits and haven't had any troubles nor have I had to remove anything from the reservoir after getting home.

You might check with your MD, but the climate is very humid & you won't miss not using water.