Denny's is perhaps not untypical - it's very similar to the old Brewtek CL-50, which had its origins in the UCD yeastbank :
https://beermaverick.com/the-complete-guide-to-dennys-favorite-yeast-strain/
Don't forget that any British brewer sends out yeast in every cask they send to pubs, so for British yeasts it's trivially easy, apart from a handful like Marstons that don't use their production yeasts to cask condition. And even with kegging, it's sometimes possible to harvest yeast that's slipped through the filters sometimes - it's certainly happened with the Adnams minikegs (although it's much easier from their - rarer - minicasks or their pub casks).
But as has been already mentioned, historically most brewers were pretty generous with their yeast, and in some cases there was so much interchange that you can't really talk of a specific brewery yeast in places like Edinburgh, they shared it around so much that effectively there was just one Edinburgh multistrain.
It's also worth noting that a lot of the "classic" White Labs and Wyeast strains came from homebrewers who had been sharing them around for much of the 1990s, sometimes via the likes of Brewtek, and so there was quite a lot of scope for them to either get mislabelled or drift away from the original genetics. Certainly seems to have happened in the case of WLP002/1968 which are fervently believed to have come from Fuller's but don't taste anything like the real Fuller's yeast (whereas eg Imperial Pub seems to have much more of the distinctive Fuller's oranginess, and I assume represents an independent isolation of yeast with far fewer links in the chain between cask/bottle and yeastbank).