Shesh,
Chimney breasts are traditional features - they give a sense of a home not an hotel room.
As for knocking thro to form one big, expensive to heat, large room - whats the point?
You might be able to see the living room lintel bearings from the bedrooms? A steel lintel is usually req'd.
Chimney stacks also give a sense of tradition and continuity.
There's a short board under the timber lintel - is there a wall under the short board?
Do you have the latest SE's suggestion in hard copy writing or in an email?
The party wall in the loft has fire /smoke spreading gaps - they must be made good.
Its hard to see, could be the pic detail on my screen but the c/b in the pic doesn't appear to go up to a stack - it seems to end inside the loft?
Does your neighbour have their entire c/b intact -ground floor to stack?
Have to go for now - dinner times up on this job.