Sorry to be thick but what's a class QA new house without losing the old one is nigh on impossible in the countryside unless you have something to trade for it. Could you get a Class Q permission on a barn you don't use and then see if you can trade that for a house of a similar size but in a more appropriate location. This has been proven in Case Law and could be an option but you would lose a barn and possibly your Permitted Development rights (although some councils do slip up).
A new build would be better at meeting EPC figures and giving you the layout you want reather than trying to work with what you have but you would need to go through the whole planning process. There are ways to get a much larger house if you play your cards right and secure all the Permitted Development extensions on the existing house then you could use that as an arguement to get more in the new build - done that several times.