I’ve been in your shoes and we built a daring extension that many architects told me would never get planning permission. In my view we built a perfect house for us. We now have tons of storage, so the house is always tidy. The downstairs layout is flexible, to accommodate ILs who come and stay often and can’t do stairs. Relocated laundry room upstairs. Built very large pantry with sink, dishwasher and second fridge. Cupboards in the hall to accommodate cricket, rugby, football etc kits, dozens of coats and shoes. Wet room downstairs with secret door to IL’s “bedroom” (which is a living room most of the time). It has ticked almost all the boxes that I had on my list.
My advice would be to actually write down a list so that you can check that your design meets it.
You’ve already had lots of comments on the layout so I won’t repeat them, except to say that the D/R with no windows will be completely overshadowed by the great room at the back and you will never want to use it. Unless your D/R is secret code for sex dungeon ☺️, please don’t keep that design as it will be poky and tight and a waste. Likewise the playroom, it’s only a few years when children want to use them and then it’s a wasted space.
Your central hallway is too narrow. With the benefit of lots of extra space, please consider how it will feel as you enter the house. You should aim for a much wider hallway that is in proportion with the rooms.
Your kitchen plan needs a complete rethink - the triangle of fridge/cooker/sink is way too wide and you will hate it. The sink works best close to the hob so you can wash your hands as you cook, drop dirty dishes in the sink etc. I used to have that layout in my old kitchen and hated it with a passion. You would be better having either sink or hob on the island. When you cook, try and note how you use the space and count the steps you have to take so you can maximise efficiency.
Oh and you probably need to budget closer to £1m than £200k.
Good luck!