How do I bathroom / kitchen / extension? etc.

Maybe this?
Not much wall stuff needs doing, and sharing a flat is way nicer with two toilets.
And I think it's a pity to hide that window in the bathroom.
Living room seems big enough to have the kitchen in there.

Whatever you do, keep the doors and frames.
And yes, c.h.e. s idea of opening up the hallway again sprung to my mind. Then use it as an American style "den", a polyvalent space. Put a desk in there, library, garderobe for coats. Lots of potential as I said!