For the OP - the porch should be designed to fit in with the existing building otherwise it has the danger of looking like a big carbuncle stuck onto the house. For that reason, take your lead from the bay windows. I'd brick the sides up to the same level as the bottom of the window in the bays then glaze with similar frames and glass. The door can be the same as what you're proposing but keep the glass the same as on your house - TBH, the photo of the door doesn't look like mirrored glass, I think it's just the reflection from the photo (I might be wrong?!?). The roof can be the same as you have and either brick the front gable or glaze it.

I think it will be at least a couple of weeks until we get our proper plans back. The house has lots of very thick walls, odd angels and a well (!) so she's still not sure what we can and can't do. But this is kind of what I'm thinking, although the layout may well change. It would prevent a kitchen window you're right...

Can you perhaps post some more (clear) photos showing more of the house? Particularly the dormer could possibly inspire suggestions.

https://dunsterhouse.co.uk/garden-rooms-and-porches/wooden-porch-half-height-w2m-x-d1-5m I don’t think a porch should cost the earth. But I think you should decide if you are just putting on a porch because there is one there now. Since you have a large plot to the side of the house you could have a garden structure there. Would some sort of gazebo get better use for the money spent on a porch? https://dunsterhouse.co.uk/garden-structures/leviathan-open-gazebo-w6-0m-x-d3-2m