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I did exactly that here: https://www.lfgss.com/comments/16283312/
I would probably be happy if I was confident I wouldn't need the cash for 5 years. If everything tanks in the first 3 years or so you know you'll have to tighten your belt a little bit to make up the shortfall. If it tanks in the last year or two before you need it at least you've had a few years of growth and you can always gradually pull some out over the last year to limit possible losses. (Just like you should with a pension.)
If you might need the money within 5 years then I probably wouldn't bother
Sure, don't take someone else's view, have a look at the historic chart of the ftse or whatever index you want to track and see how often it is below the level of was x years ago.
Five feels a bit risky but I've not done the analysis.
Personally I feel the market is a bit toppy given the shit macroeconomic outlook and would be very wary of investing a lump sum now. But in not looking to do not done the work on it.