I'm American and I came back here after a 4-year return to California years ago (I'd already lived here for a couple of years). What I missed then is what I would miss still and came back for:
The greenery/countryside/parks
The ever-changing skies, the light, the wind
The safety in knowing that a great cup of tea is always around the corner, hail, rain, or shine. Which leads to:
The tea, Tunnocks, sausages, the fish, the potatoes, the biscuits, Heinz beans and also tomato soup, British apples, all the food, really.
The humour and you guys absolutely still produce some of the most watchable programmes ever! I'd miss the telly, for sure.
The different accents.
The geographical closeness to so many amazing places on the planet: The beautiful British isles themselves, Europe, North Africa, Iceland.
The British love and respect for animals, especially dogs.