We live in a 1930s detached house, and are planning something very similar.
We are looking at removing the chimney breast from the 'middle room'. Each of our builders have emphasised that it is very important that steels are inserted to support the remaining parts of the chimney breast (i.e. those in the bedroom above and the loft). Without this apparently the whole thing can collapse.
We have opted for a utility and toilet in one room, with a door from the hallway. I'm happy with the door there as 1. Laundry comes from upstairs rather than the kitchen, 2. I don't fancy the toilet door opening into living areas- the hall feels better. We don't envisage using the loo a great deal- it's more for when our increasingly old parents visit, so I don't have an issue with the two rooms being combined- the advantage being that the kitchen is larger.....
We also intend to block off the front room. Currently everything is open plan, and we would like to have two separate areas. We find it tricky with teenagers' friends visiting. Plus if I have a friend over for a coffee DH and DS can hear our conversation.....
The front room will be quite small but think we can work with it. I love the idea of sliding doors to the middle area, but think that might take up valuable free wall space and make placement of the sofas difficult.
We are currently thinking about bifold doors with integrated blinds at the back of the house , plus three velux windows. I agree that the middle area may be rather dark unfortunately. Hopefully the velux windows plus the window at the side return in the kitchen area will help.
Sorry for the long post- it's very exciting to read that someone has similar plans. I've been trying to find images of this type of set up, but haven't had much luck.