Just looked up speckled sussex 
that colouring! So pretty!
I don't have much experience with cold climate birds, as I live in the desert, but I think all these breeds should be okay. I generally make my breed choices off of personality, eggs, and appearance (in that order).
For brown eggs, I love black australorps and gold sex links. Every one I've had has been very friendly and bold. My current sex link has almost no fear, just curiosity, and is constantly following me around the yard. So far, I've found that behavior to be very stereotypical of the breed. I don't have any australorps right now (even though they're one of my favorites), but they are hardy birds with small combs and some meat, so they should fare well in cold weather. They are super sweet, too. Both are great layers.
Marans and Welsummers should give you darker brown eggs. I've only had one of each, so I don't know if my bird's personalities are stereotypical of the breed or not. They're beautiful birds, though, and I get a pretty steady flow of eggs from the both of them.
For green eggs, I was going to recommend Easter Eggers, but I see that you can't find any in the UK. If you do come across any, or if anyone else is looking here for suggestions, they're one of my favorite breeds. My current girl gives me about 5-6 eggs a week, all a nice shade of green. They come in all sorts of beautiful colors and are pretty gentle. It should be noted that, in my experience, EEs are very intelligent. This is definitely reflected in who figures out how to get into the garden or treat bucket.
Hope this helps a little! Good luck!
really clear detailed post 
really helpful!
all we get is rain! 
🌧 apart from when corona virus lockdown occurred and we can't go outside ... now all we're getting is sunshine 
☀
ordered some french black copper maran hatching eggs to hopefully give me some chocolate egg layers 
but the UK don't seem to have the same chicken breeds as in america we have all the british breeds, but little else.
I have to say personality wise, my speckled Sussex takes the cake! She is part of my trio of one year olds, the Sussex (colorful with big light brown eggs), a silverudd (splash with lovely green eggs), a welsummer (skittish but such beautiful dark brown eggs!)
Haven't heard of silverudd, but just tried to find them in the UK and no luck!
They are frequently called IsbarYou've converted me to the sussex! They sound lovely (they've been recommended to me several times now, so they must be good)
Yeah the Easter Egger are gorgeous and the eggs omg all kinds of different color, green, blue, pink, all kinds, the little batham I had was gorgeous with nice hair stile looking head, but she was so noise that I had friends who could hear her from a block away from my house.I wish I could get an easter egger! I've looked so hard for them, but they're impossible to find in the UK
Are domnique's similar to barred plymouth rocks? I think the main stock used in the development were domniques (hatched some rocks, so hopefully they will be as sweets as your domnique!) 
