Re the lounge area don’t forget it will not have any natural light once doors are closed, it looks quite dark already although that may just be the photo. Definitely consider some roof lights in the roof section that is replacing the conservatory as that will bring more light into the dining area and the lounge through whatever opening you choose. Maybe use glass doors between lounge and diner?

I had all the electrics re-wired in my 1934 house which involved a lot of channelling of the walls. The electrician then filled the channels a little and then I had a plasterer smooth it all over and make good. He did an excellent job. What makes you think it’s a lathe plaster? I don’t think it would be a problem for any decent plasterer. I would get some quotes and discuss your worries with them.

You really need to get a trader out to quote you. The quote will vary according to where you live.

Do you need to keep a downstairs bathroom, or are you happy to have only the en suite until the loft is done? (bearing in mind this may put buyers off if you end up selling before you do the loft). If you do then I agree with SpaceMaker’s idea of moving the bathroom to the dining room area if soil pipe allows. Or if soil pipe does not allow (eg if soil pipe runs back through the garden rather than under the house) then at least move the shower and sink, so as to leave only the loo carved off the kitchen. The dining room could divide into a bathroom and utility on the left hand side (using the under stairs space as well) and then a corridor through to the kitchen on the right. If you decide you don’t need the downstairs bathroom then you could have the same sort of division but make it a utility and study space. You will free up quite a lot of space if you take out the chimneybreasts - in your shoes and budget allowing I would keep the one at the front so you can have a nice fireplace/woodburner as a lounge focal point, but then take out the others. Certainly a side return extension, with glassed over side roof and glazed doors to the garden at the end, would have a big impact on the size and feel. Rooflights are cheaper than a full glass roof and have the benefit of being openable for ventilation in the summer.