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Outside of my expertise :|
Is it the pic at the top of the page - is it a small bit of flat roof at first floor level?
Simplest might be to find a product you like, find out whether it would work (size and what it's fixing to) and quote for fabrication and install altogether, they will do drawings etc and they should be able to provide info needed for building control. But I don't know if you would need it - maybe not if it's considered a replacement. Phone call to local authority building control or a private one should tell you.But basically design and fabricate =/= pick a product, and the latter is easier if you find one you like that works.
Unless it's got some historical/conservation get-out, at the very least you need to make sure the replacement is up to regs.