When I lived abroad it used to be:
the higher standard of driving,
good manners in general, especially with queuing and personal remarks/questions in social situations,
Customer service and efficiency,
British supermarkets quality of food and prices, especially Tesco. I’d be like a kid in a sweet shop on trips back home in there marvelling at how cheap everything was!
British chocolate
Sadly all the above has gone to pot in the years after we returned to the UK.
A great British roast dinner with Yorkshires and gravy. As well as a cup of strong builders tea and buttered crumpets.. My attempts to recreate those were never the same when living abroad.
Vegetarian options (I’m a veggie). May have changed now but I never found a country other than the UK that offers such a good range of meat free protein replacements and veggie dishes in supermarkets and restaurants. Really struggled with that.
I remember while living in Western Canada and Malaysia desperately missing the fresh summer breezes that we’d get in the UK when the heat there was stifling and there was no let up day or night.
The relief of the friendliness and professionalism of passport control on entry back into the UK, especially on the Eurotunnel after driving through borders in Eastern Europe, when we lived there for a few years. Not really something you miss until you come back!