You have received a windfall ...

Air conditioning units installed throughout my home. Last summer in London, and for several summers in London, the heat has been oppressive and has affected my wellbeing.
Am doing this in the New year anyway, will cost a chunk of our savings, but I think is now essential.

Fallingstar

Air conditioning units installed throughout my home. Last summer in London, and for several summers in London, the heat has been oppressive and has affected my wellbeing.
Am doing this in the New year anyway, will cost a chunk of our savings, but I think is now essential.

That is a good one.

If my windfall wasn't enough to install (and run) good air conditioning, I would prioritise having external awnings installed above all the windows that have the sun on them at some time of day.
If it was big enough, I would have both.

Air-conditioning is good future proofing. My first thought was teeth - but I was in the dentists chair when I thought of the question

Another thought has been wooden floors to replace carpets. My back really doesn't like hovering these days and I doubt it will get better as the years pass.

A new ceramic hob. Mine is cracked and I’m saving to replace it.

New kitchen floor, torn when the old washing machine was removed. It’s nit dangerous, but unsightly.

“Net curtain” blinds for the living room…

In my current situation and state of health, a measly few thousand wouldn't cover it.

In these uncertain times, I'd be inclined to buy gold I could wear on me... not many scenarios where that can't be exchanged for what me & mine need to get along, whatever that may be in 20 years' time.