Turning an HMO into a family home
I used to live in a house with a dining room that had no outside windows (Also in the UK).  Similarly to yours it had a kitchen extension behind it (SW-facing.  It had a big window between the two, and a partially glazed door which we replaced with a fully glazed on (lots of little panes). At the front we had a slight advantage over you - the dining room was semi-open-plan to the living room (it had a half height wall).  There was an RSJ as the original wall had been taken down.

The dining room was very dark when we moved in.  Just replacing the dark wallpaper with pale paint in the dining room itself and the lounge made a huge difference.  Improvements we made to the kitchen helped too.

In your case, I'd look at:


Removing walls that have been added (e.g. probably those between bedrooms and their en-suites)
Adding internal windows between the living spaces.  In fact it might be a matter of restoring original windows in the S (right) wall between the DR and lounge 2, as that was the original external wall, though L2 might have been converted from  garage to a bedroom given the size and location.  Depending on how much light comes into L2 these will make a big difference (while L2's windows face S, they're neither front nor back windows so could well be shaded by next door.
Adding internal windows/glazed doors (maybe double doors) between the dining room and hallway. I wouldn't knock the wall down even if it's not structural:  A front door and stairs in the living/dining room can be perceived as belonging to a cheap house, can be draughty, and with your open-plan layout into the kitchen could lead to cooking smells  in the bedrooms.


These ideas would be a little cheaper than yours while still opening the house out into something that feels more like a single home.  

On thing to watch with HMOs is that extensions may have been built or garages converted on the cheap with little insulation in the right place, so before you get too far you should check.