Distilled Water for C-Pap machine where to find it

I'm a CPAP user.
Consider some of the ideas in this thread:
http://community.ricksteves.com/travel-forum/england/cpap-users

De-ionised water is not distilled water.

You won't find distilled water. Tap water will be fine for the brief time you are in the UK.

Plenty of online suppliers, but hard to find in the High Street.

My husband just went through this search in the 4 weeks we just spent in the UK. His most trusted locations for finding it were at hardware stores in the automotive department. They all knew what he was talking about. In the hardware stores it was called water for topping up (battery related). We spent 2+ weeks in one location and 1+ weeks in London. When he found a source and we needed to change locations, he would pour the large container into 2 small empty water bottles so he could transport it to our next location so he wouldn’t have to repeat the search. Also he brought a small bottle of distilled water with him in his checked bag so he wouldn’t have to search the first time in an unfamiliar location. In our first city we went to Tesco, Waitrose, Sainsbury and Boots but they didn’t carry it. Finally someone suggested a hardware store automotive department and that’s where he was successful.