Loving the idea of a welded frame - I would imagine it would stand up to our winds really well! I doubt I will be able to follow suit with my first effort, but I will be taking inspiration from some of your other ideas.
I'd already decided on a similar roof design - our garage has a corrugated roof with clear corrugated skylight in it - HUGE savings on electricity costs and lets in loads of light, even though it is probably only 1/9th the square footage of the garage roof - I'm thinking of using the same materials for the coop roof.
Let us know how the ventilation works out with it, whether you think it is enough for the Welsh climate etc or whether the drafts are too much? I was thinking of adding some sort of edge underneath my roof to minimise the biting winds we get, and then maybe some additional vents somewhere else in the structure that can be opened/closed.