alt Hacker NewsBoots (a large UK pharmacy chain) sells it, but it's "behind the counter" so you have to ask for it. They will also only sell you one pack of 12 per visit
It's the same for codeine-mixed products. Recently I got denied a simultaneous purchase of pseudoephedrine and ibuprofen/codeine - I was I was only allowed one.
I'm not sure if it's policy, law or I just looked suspicious.
Thanks. Looks like the buying limits are a legal requirement: https://www.gov.uk/drug-safety-update/pseudoephedrine-and-ep... ) Seems like it was also classified as Pharmacy only (behind the counter) or prescription instead of just OTC.