Kitchen design apps??

Skylight, flat lantern, like an orangery roof, image attached?, or pitched like a velux?

So Double doors onto the garden, 1800-2000 width?, are you thinking closer to the dining area so you can see the garden? this would give you more surface potential for the kitchen, if the patio door is not symmetrical in the extension wall.

Your ceiling height in the existing room looks lower than 2200mm 7'3", it looks more like 2000mm, this might be a trick of the eye mind. 

Have you thought about adding an Aga electric module onto the side of the Aga, these do come up now and again on ebay around the 1500-2000K mark, though it would mean moving the existing aga over as I'm sure these are always on the left-hand side, not the right.

Cabinet heights, you mentioned you were short of stature, that Aga of yours will be 900-910mm off the floor but, your Island could be a little lower if you wanted, this only requires the legs on the cabinets to be wound down, so you could reduce this to 860mm if you wanted. It's a lot easier to roll pastry or icing a little lower than standard worktop height.
Talking of heights, there are three heights of tall cupboards these are standard and not special sizes. 1970mm floor to top of the cupboard, these are classed as low larder cupboards and suit a wall unit that is 575mm high, 2120mm which suits a 720mm wall cupboard and 2300mm which suit a wall cupboard which is 900mm. You normally have a gap between the worktop and wall units around 490-500mm dependent on the thickness of worktop. Though there is no legal requirement so you could reduce this is you wanted, and have a stepped height. 
As your property is older it tends to suit a more eclectic style with mixed colours and potentially mixed worktops, so you get a more freestanding feel, and if there are wall cupboards they are not planned wall to wall instead those cupboards they have spaces around them, like the images attached. 

Fashion in worktops is getting thinner, so 20mm or 12mm is a lot more normal for solid surface worktops these days whereas traditional laminate worktops were 38-40mm thick.

I'd expect you to have 300mm nibs to support steelwork beams, so the one between your window and patio doors will be 300mm deep and 500mm width, it's the size of the small return on the opposite side by your existing front door, I'm guessing this is about 360mm based of 15' feet and 13'10" dimensions.

I've also attached a basic plan of the space, mocked up on my system, does the door into the rest of the house, have a recess as drawn or is it flat to aga wall.