Your house looks a similar dimension to my house which I just sold. I kept the wall dividing the receptions rooms but took the wall down between the kitchen and back reception room. My kitchen was in a little out rig (original not extension) but i think you could do something similar? https://www.rightmove.co.uk/house-prices/detailMatching.html?prop=50634231&sale=5936314&country=england

I'm in Jonathan's camp re splitting the lounge, I'd also think before doing it about your lounge furniture. The wall that you are thinking of having a hatch in looks like a prime sofa wall. Therefore, where will you put sofas? Also, why would you want a serving hatch going through in to a lounge anyway? They traditionally go through to the dining room, which, here is not the case. Not sure why you would want one on that wall, and what you would be passing through it? I don't like to pooh pooh peoples ideas, but it does sound just a little odd. I think that a lot more could possibly made of the space in general, with the help of a good concept planner maybe, as they're worth their weight in gold for using all available space to the utmost. Here's one idea:- Bearing in mind it's not to scale, just 'an idea' .

Yes we needed an RSJ and all of the walls underpinning as there were no foundations. Was very costly but definitely worth it as we live in that end of the house. I think that With having an older property, costs do just spiral. I can’t remember how much the RSJ was, sorry. i think our building work was about £25000. Can you leave the fireplace in and have it open so you can see it from both sides? Then have your kitchen and dining in the current lounge and your living area where it is at the moment but open it up apart from the firePlace? We’ve left in our old exposed stone work and cleaned it up.