Hello. =) For what it's worth, when me and my husband (we're from CA) went for 16 days in 2019, we did it it like this:
8 days in London which included some day trips to Windsor, Cambridge/Grantchester (we rode bikes from Cambridge to Grantchester where they filmed the show "Grantchester"), Salisbury/Stonehenge/Old Sarum, and Corefe Castle/Swanage (this is on the southern coast- there was a Pirate Festival that day LOL).
Then we moved on to Moreton-On-Marsh where we stayed one night and went down to Stowe-In-The-Wold and Bourton-On-Water (two nights would've been better); then we went to York for two days with a day trip to Rippon/Fountains Abbey (ruins) and day trip to Retford for a Pilgrim/ancestor tour. Finally we went up to Edinburgh for 3 nights. We flew from Edinburgh to Heathrow and spent our last night in London before flying back to LA.
We did our trip by sections including London to maximize our time and we didn't rent a car so we used the Tube and trains and walked. I'd say all we did was totally worth it as it was our first trip to the UK. We had a FANTASTIC time and had no bad meals, I'd like to add (did research on where to eat ahead of time for options).