In reply to Flinticus:
What's wrong with the ones you have, are they rotten, draughty, stuck, or is it just to get double glazing?
You can get standard* sealed units down to 14mm thickness, likely to necessitate some modification but unless they're extensively rotten it's probably worth modifying and refreshing what you have vs buying new especially given today's timber prices. I'd find a carpenter/cabinet maker or specialist window restorer, see what they think.
New sealed units for that lot will leave you change from £1k, that potentially leaves a lot of budget for restoration and modification.
*as opposed to exotic vacuum 'filled' things.
Looks like a scaffold or cherry-picker job?
jk