ooh, how fun - I hope you enjoy! I have never been to the scottish countryside but I love edinburgh, it's so beautiful while also feeling so real and lived in.

I personally love london but I think I love living in it more than touristing in it, and if you've seen the main tourist spots already I'm not sure coming to london is the best use of your time. especially with how expensive london hotel prices are! but if you do find yourself coming to london let me know, and I can give you tips on seeing it more from a local's pov, especially the areas around the heath. (I'm glad you felt like I described it with warmth because I do love the heath so much!) there are also smaller towns that might be worth a day trip from london, but probably not a trip especially (rye, whitstable, margate, come to mind).

as for other places, it depends on what you want or like! for nature/beautiful scenery, the lake or peak districts are both great options, but a bit difficult to get to unless you are able to rent a car and drive. an alternative is devon/cornwall, but that's basically about as far as you can get from scotland. st. ives is kinda touristy but it is truly so cute.

for more beautiful old picturebook cities, I love york - and it's not too far from edinburgh. also cambridge and oxford (I personally prefer cambridge - it's just a town and so has much more of that pretty old-timey vibe because the roads are smaller, all the old buildings are closer together, etc. but I may be biased.) I've never been to durham but it also has a collegiate style university so it might have similar vibes? it's definitely a proper city, much more so than oxbridge though. I also love bath a lot (went there to celebrate getting engaged! jane austen hated it but she was wrong), but if you've been to stonehenge you might have already seen bath.

I hope that helps as a jumping off point! let me know what you end up deciding on and how the trip goes!