Personally I would move sink off island and put it on rear bench

Now I have my glasses on the study space looks too narrow to be useful for anything. How wide is it?

I’d consider having a hinged door to the ensuite instead of sliding and moving the toilet to behind the door, otherwise I think your plan will work well. Personally I like to have the sink in the island. The study is probably too narrow to have a bench and reasonable walk space

Agree with the bathroom changes. Hinged doors on both bathrooms easier to lock and better for noise control and easier to lock
If u move toilet in ensuite you can towel rail in its place and a walk in shower
I would also try and get a powder room Even if you take the toilet from main bathroom it will enable someone to use loo whilst another in shower

Personal I would build a second storey rather than have no back yard. Imagine if everyone around you covered their lots like you. No sunlight no garden space for trees or veggie garden. Where will your storage be for bikes etc. guess you want need much gardening equipment

You need windows to kitchen and dining

I'am wondering why you have placed the kitchen on the Northern side of the house blocking that Aspect and sunlight.

I too am planning an extension. It’s exciting but time consuming. Is the existing kitchen, sunroom, bath in an extension already. What is the floor and roof? Will this be ripped down and rebuilt? Did whoever drew up your current plans do any investigation of the condition of the current building? I ask because joining a new extension to an old extension can be expensive way of building.