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If they were sash I'd say leave them as they are and let someone else pay to restore them in the future. If you sell up next year or even the couple after that I doubt you'll see a £20k return on the cost of replacing them. So I'd guess it's down to how much will it improve your life within the next 3 years to get them replaced now vs how much of a hit is it to lose 20k from the savings account.
No, they're casement windows. Timber-framed but not nice-looking or special enough to try to restore as far as I can see.
If they were timber sash windows I think we would go down that route.