I'd start with the old fashioned work out on paper -- income/outgoings, remembering to allow for the occasional little treat (getting hair done, eyebrow-lip wax , house insurance, heating, newspapers etc. etc.
Then think of how to cut corners. 90% of my clothes come from the clearance section of an M & S Outlet Shop, Charity Shop, or eBay. I never ever pay full price for anything, ever. Books from the library or on line for an Kindle.
Anything bought on eBay that doesn't fit can be resold.... if you want to be bothered with this. And I have had a great eBay clear out of unworn clothing/shoes of my own and of my dh.
Grocery shop on line, in Asda it has to be over £40 for a home delivery. It's possible to choose a slot for £1 or £2 if you don't mind it coming at say a 7pm slot. I find that by grocery shopping on line I spend much less.
With no car, no mortgage (I'm assuming also no rent) and no debts you are on a head start.
Being retired means less outlay, no transport to work, lunches, no giving to leaving presents and not needing work clothes. Definitely less clothes needed.
If you think that a major spend is due on something for the house do it now while you are working or at least put some cash to the side ear marked specifically.
If your £12,000 income is nett I would think that with careful planning it would be possible. Do the debit/credit list making it very comprehensive and see what you think.
When I was on my own I coped on much less, about £900 nett per month and also managed a runabout car/petrol as my mother lived 20 miles distant and needed help.
If you can get your figures to work out in theory you can then refine.
I hope it will be possible for you, I have loved being retired.