I looked at this very kitchen but instead went for the Wren equivalent which is also timber framed....the designer range Shaker kitchen. My builder commented how it was more solidly constructed than the Howden kitchen and it came ready to fit (not flat pack) so was easy to install. Pic below. The island is green but the rest is off white. They will match or better any other quote. Their designs were also better than Howdens.

What material is your countertop? In general I don't see a problem with the tap being on the counter (just make sure that it actually reaches as far as it needs to!), but if it is a wood worktop then you might consider having a piece of stone there; wooden worktops can show water damage fairly quickly, and it's not always feasible to remember to wipe it after using the tap each time!

I see that the neighbours' extension doesn't extend 6 meters all the way across, but lines up with your 3 meter extension where the two houses meet. At the least I'd consider doing something similar. If you add a further 3 meters all the way across, you will be creating a very dark little courtyard for them which I think they'd very likely object to. Keeping your internal floor levels the same will mean that your decking area will be very high. Difficult to tell from the picture as I don't know if the garden's slope is exaggerated, but it would be a good idea to measure the vertical drop from the internal floor level, to the point where the end of your decking will meet the garden. That will be the height of your decking from the ground. And consider whether it will in fact cut you off from using your garden. You may also need quite a bit in the way of retaining walls which are expensive to build and clad (I speak from direct experience - we are currently building a 4m extension plus 3.5m terrace on a garden which slopes downwards. The retaining walls are costing an arm and a leg. We have a couple of steps down from the existing house into the extension, but the end of the terrace is still really high up from the garden). Just some things to consider. I assume you'll be engaging an architect and if not, I'd strongly encourage you to do so - they may well come up with some ingenious solutions to create extra space in a way which doesn't impose on the neighbours and still allows you good access to the garden.

Our local authority makes you divide it up, so that everything directly in front of A is measured from A and everything in front of B is measured from B. doing a wrap around extension requires planning permission if you want to go more than 3m in front of A. Does that make sense? I would add that the drainage for a Victorian terrace often runs really close to the end of the house at B. If you are thinking of extending there scope out where the drains and manhole covers are as that will affect the complexity and expense of your options.