I don’t know, to be honest I’d be pretty enraged if I can’t get my planned holidays this year for any reason, the hotels being stuffed full of pensioners being one of those! To mitigate against this, I’ve made a load of refundable block-bookings, which also gives some flexibility regarding weather. I’m not sure there will in any case be that much demand for holidays in Swansea, Port Talbot or Newcastle...
I suspect it’s unlikely hotels will turn away unvaccinated people. Judging by what I’ve seen on a few hotel Facebook pages recently, and from conversations when I was in Wales last October, I don’t think hotels will be wanting to restrict people. But wait for the pensioners to pressure for unvaccinated people not to be allowed in places that they wish to grace with their presence. I bet we’ll see at least elements of that.
I expect that so long as we have relaxation similar to last summer, hotels will be taking bookings from anyone regardless of vaccination. The limiting factor will be how safe people feel. I doubt that pensioners will be as vociferous or organised as you fear on this respect.