If you were going to the effort of plastering over I'd just remove the stone altogether. Least then you gain some space too. Some people love this sort of thing, some people hate it. No one likes a bodged cover up job.

Just make sure you have thought through it properly. Full glass wall along would mean you couldnt have any furniture at all anywhere along that wall, or at least not the last 80cm of length before the doors. It would make the room much colder - vertical radiators (taking up more wall space), underfloor heating? Just a thought - you have a huge expanse of glass/windows at the moment (way more than the average lounge) and you say you don't feel you get a lot of light in....... not sure if full length doors would make that much of a difference. Maybe its more to do with the position of the house.

Good morning Hazel, Here's a link to a guide on small extensions under permitted development, if you haven't already looked at. https://interactive.planningportal.co.uk/mini-guide/extensions-single-storey/0 Once you have more idea on the construction of the extension and the exact dimensions, you'll be able to properly assess the internal space you have. You are correct in saying that it wouldn't be ideal to have to walk through the living space into the extension for the dining room. The proposed extension itself may not be ideal for a living/family area once a sofa and TV are in, as this won't leave much space for an external bi-fold/sliding door. Another option to explore is moving the kitchen to the left and having the living/family room to the right hand side (where the kitchen is currently situated). This will then mean the extension will hold the dining space.

Wider tables are great. My table. is 120 wide, it fills the room nicely and more importantly there is room in the middle for serving plates. You only feel like you are further apart because you are not used to it because generally restaurant tables are small in order to fit more in