Short answer: it depends.
Longer answer: it depends on the university and the funding body. Not every offer of a PhD place comes with funding, although many do.
I'm doing a PhD in a humanities sort of subject at a London uni. The other students and I have some form of funding, along with a separate tuition fee waiver. I have seen stipends of 8k, 14k, and 16k. Some students are required to teach by their departments, although this varies. I myself would only get paid if I did above 6 hours a week of teaching. My funding is tax-free, which is nice.
I do know one fellow student who was offered a place at a very good uni, but turned it down as it had no funding attached.
Living on a PhD stipend doesn't give me much room for fun. I don't eat out much, don't buy clothes unless something has a hole in it, don't go on foreign holidays. It is doable though. My own financial position is fairly secure because my partner works in the City and earns a good wage; I do pay a proportional share of the bills and such out of my stipend.
Can you afford it? List out ALL your expenses as a couple, being honest about how much you really spend and not forgetting things like gifts, drinks on a night out, and a bit for emergency savings. Maybe you can. You will probably have to cut down on some things. Maybe you don't get that new outfit for someone's wedding, maybe you have to shop in a different supermarket now.
PhDs are hard work, but may very well be worth it.