Unfortunately I bought a brand new Oral-B iO shortly before I started boycotting American products and now I'm kinda stuck with it. I don't want to waste it, but at the same time I'll have to keep buying their expensive brush heads.

I do at least use Sensodyne toothpaste.

Morrisons is owned by Clayton, Dubilier and Rice (CD&R), an American private equity firm.

"It's always in the last place you look"

No shit?

As the other person who replied to you said, Sodium Lauryl Sulphate (SLS) is likely the cause, it was for me. Sensodyne do a Pronamel range which doesn't contain SLS and I've been using it for a couple of years with no issues as a result.

Apparently two brands make them, Cremodul and Trustnice; maybe a few others, but I'm not keen on using knock-offs because my teeth are quite fragile and apparently both of those brand knock-offs are much harder on the teeth. It also appears you can only really get them online from Amazon, which I'm definitely not keen on - out of the two, I'd rather throw extra money at Oral-B than a single £1 at Amazon.