This really shows why consulting with architects is worth the money, anyone can throw an extension on but this is lovely and improves the look as well as space

If you're leaning more towards a pitched roof, perhaps you could extend your glazing right up into the gable to maximize the light coming in. I've attached a couple of images of projects where we have done just that. The first was necessary due to it being a listed building so we suggested an additional tiled roof would be more in keeping than, for example, a flat roof with a roof lantern. The second was on a lovely Victorian terrace in Coggeshall and again, we suggested echoing the original architecture with a pitched roof and glazed gable. If you would prefer an orangery style extension, despite some previous comments it is definitely possible to have a space that is a nice even temperature all year round, doesn't collect lots of flies and doesn't leak. We have built these for 30 years. All of our roof lanterns are available with solar controlled glass and motorised vents, plus the combination of masonry and glass (as opposed to a traditional conservatory which is mostly glass) provides a a more consistent room temperature anyway. We offer a 10 year guarantee on our joinery, glass and paint coatings (parts and labour) - that's how confident we are that it won't leak. We also use Accoya wood in our products which is incredibly durable, stable, moisture resistant, fungus proof, inedible to insects and thus incredibly low maintenance. It's definitely worth googling Accoya if you're considering going for a structure that incorporates timber joinery.

Agree with Jonathan, it would be a shame for the view from front door to be into the WC. And also better to have the WC on an outside wall (add a window to that room in Jonathan's plan?). And the walk-thru utility/laundry room is really useful as the day to day entrance to the house and removed from everything else. The only change I would make is to reduce the size of the front living room a little - it looks too long for the width of it and the back snug area would benefit from that extra space.

The best thing would be to use a Party Wall Surveyor - there should be a bit on the form to say you'd like to do that, but if they are actually nice people it would be nice to hand-deliver it and also have a chat and just explain your concerns and how you'd feel happier knowing it was all being dealt with by an independent person. It will probably annoy the neighbours a little (or a lot) and may delay the start of their build (or it might not) but it will definitely save you any headaches over disagreements. You can each have different surveyors or you could use the same one - the surveyor is acting on behalf of the wall (even though the neighbours doing the build usually pay for it all) so should be independent of anybody involved and just makes sure everything is done properly. You could suggest using just one surveyor to keep costs down, to demonstrate you are a reasonable person. One thing to be very clear on is where they pour their foundations - none of it should be over the boundary line - if they are building a wall right up to the boundary line then their foundation needs to be designed in a way where it's all under their land, but the wall sits off-centre of the foundation. The foundation design should show on the building control drawings (structural drawings) - maybe ask to see those drawings to check what it looks like and if it looks like the foundation (the concrete slab) will be underneath your land then tell them that's not allowed and definitely use a party wall surveyor. A decent architect/surveyor should have picked up on that already, but someone might be chancing their luck on it - and if they're chancing their luck on that, then I wouldn't trust them to do anything properly so would definitely go with party wall surveyor. If there is foundation concrete poured below your land then it is a trespass on your land and it will impact you building your own extension in the future - either you'll have to dig it all out (and then they risk their extension not being secure) or you have to step your foundations (and extension) in away from the boundary to avoid their foundation. Either way, this can be avoided by appointing a party wall surveyor now, looking at the structural drawings and keeping an eye on the builders when they start digging as the guys doing the actual digging might not know where the boundary is. Note that the builder is offering to take down your WC now as it'll make it easier for them to build the neighbours extension - not because they are being generous. I would not take anything down now, wait until you are doing your extension. Good luck!