Water for CPAP in London

My husband used tap water with no problem while we were in London last May. We also saw many pharmacies, so I think you'll be covered either way. I suggest doing some online searches for pharmacies close to where you are staying, and then search their websites for what they stock.

My husband just uses bottled water in his CPAP on multiple trips to the Europe and the UK with no ill effects. Don't worry about this.

As others said, just filtered, bottled water will work fine. Distilled water is preferred for long term use since it eliminates any chance of mineral build-up in the unit, a few weeks of purified water will not affect it in the least. Of course checking with your specialist or provider might be prudent.

When I was in London in February, I went with tap water and saw buildup in the water chamber starting within days.
I told my daughter (who I was visiting) i NEEDED for find distilled somewhere. She was stumped as to where.
One evening we were shopping at a big Sainsbury's and happened to see distilled water in the automotive section. Smaller bottles (maybe quart sized) than we see in the US (gallon size).

That's what I'll look for next time I visit

Yes, distilled water should be available at motor garages, or at pharmacies.

When I was in London in February, I went with tap water and saw buildup in the water chamber starting within days.

That "buildup" is just minerals from the water that have dried in the reservoir. Just take a clean towel or cloth and wipe it out—easy peasy. I do it all the time when I travel, and have never had any problems.

The trick to using tap water in the humidifier chamber is to empty it, rinse it out and refill it every day. That basically eliminates the buildup of minerals.

Water in a CPAP isn't therapeutic, it's for comfort. Some of us don't need/use water in our machine.

I've read that Sleep Apnea is just as common in the UK, however it's not tested for and the CPAP fix not often offered. Hence the trouble finding distilled water.

CPAP machines are available through the NHS in the UK on prescription from a GP / specialist sleep clinic. However, these often don't include a humidifier as there is less routine need for this in the British climate. They are more of an add on for those who find they need them.