Some cafes will do it - not as standard, but a few - maybe try the ones trying to be 1-up from a greasy. https://seahousescafe.co.uk/the-breakfast-menu
As will many hotel breakfasts, there's often little single serving marmite things in with the single serving jam packets. I'd say about half the hotels i've stayed in with decent cooked breakfast have had it on offer.
I've also seen it in little roadside food van / trailer type things too.
Anyway, you want sainsbury's yeast extract instead of marmite, it's way gloopier and nicer tasting.