The union will presumably fight for their members (and rightly so), so this could put you at a disadvantage, but otherwise there are rules that employers have to follow when deciding who is selected for redundancy. They can't decide to choose younger people over older ones, so there is no saying that the axe will fall on you. If you would be retiring soon(ish) anyway, it would be cheaper for your employer to keep you on until then, as you won't get a redundancy package when you retire.
A year's untaxed salary will be a lot higher than 12 months' take-home pay, so if the worst happens you will find that this, coupled with the fact that your expenditure will go down, will mean that it lasts a lot longer than you might expect.
When I left full-time work (at 58), I saved a lot on commuting charges, and even things like cups of coffee, lunches and so on soon added up (or subtracted themselves!) and I was surprised at how much less I was spending. I didn't need to buy work clothes, which also saved quite a lot. I stayed active in my professional body, and was soon offered a bit of consultancy work. It is not a lot, but keeps my face known, and brings in a bit of extra money. Maybe you could start letting it be known now that you might be interested in little bits of work down the line. Do you have a Linked In account? If not, you could start to set one up now, so that you have recommendations and so on before you leave work (if that's what happens).
It is worth calling the pensions advice line, and checking whether you will have a shortfall on your state pension - the odds are that you will as most people in the civil service were 'opted out' of the pension part of NI contributions - and ask if it is worth buying back the missing years. You can do this in quarterly payments if it is worth your while (and if you can afford it). You are gambling on living long enough to get back the money you spend, but if memory serves you usually break even four years after retirement, and after that you gain quite a lot.
I hope it works out for you, whatever happens. Some of it is out of your hands, but if you can start planning for a worse case scenario now, you might be able to mitigate at least some of the financial impact if it happens.