I would consider what was most secure in your life and go from there.
Do you already own property? Could that be lived in in old age? If not, that's the first thing I would try to solve- by getting a one/two bed flat somewhere not amazing but ok, because I am not at all convinced that the state will be paying out housing benefit in 20 years time to all us oldies.
If you do own property, can you rent out a room? Lodger with cooking/extra facilities? I know one older lady who rents two rooms out, both rooms are self-contained but they share the bathroom. This is in a desirable area of the country where people are seeking Mon-Fri work though. Not everywhere has lots of people looking for lodgings, though I would guess most places have.
Then I'd see what pension provision you already have and pay NI as others have said.
Then I'd think about how you want to earn your money daily for the next 20 years- you don't want to be using capital to do that, so some of the suggestions here are good: cleaning, childcare, renting rooms out, working as a receptionist, starting in temp banks to begin, just to get something on the CV.
I would have a small amount (e.g. £500 a year) for treats- a haircut, charity shop some clothes (I find really nice ones), the odd cheap meal out, go and see friends, anything to make daily life more fun. My mother is not wealthy in old age but has joined clubs and gone on some really cheap but fun trips (coach, music type things) in great places.
This won't solve all your money problems for ever, but it's an amazing step up at just the right time for you.