I'd ignore the stupid regulations and just get someone to do it. You
are only up-rating an existing window not installing new - even if
up-rating involves replacing all of it.
Any future buyers are unlikely to be bothered in the slightest as long
as it is a reasonable working window and not a conspicuous bodge.
Talk to several joiners and see what they say - avoid DG salesmen who
are all cowboys.
As for the design the possibilities are endless - you can have what you
like as long as it is practical.

cheers

Jacob