I've lived overseas for a few years now (Africa - including some pretty bad dictatorships!, North America, Europe), because of mine and DH's jobs. We enjoy it - have young dc - for now, but the longer we live out of the UK, the more British we feel, and the more we appreciate being British, and miss our country.
Just a few that come to mind for me:
Free point-of-service healthcare via NHS
Family and friends
A sense of easy belonging
The BBC, and other UK TV
Free (and vocal) press
Beautiful cities packed with historic buildings
Stunning and hugely varied countryside & coastline: from mountains to lakes to ancient forests, golden beaches to dramatic cliffs, rolling farmland and craggy moors.
The UK is small enough to travel around easily, affordably and frequently
Democracy
Human Rights
Justice system
Welfare state/social services
Multi-culturalism
Political correctness
Diversity
Tolerant, charitable people
A great, far-reaching and free education, given at mostly very good, free schools.
Universities (they're the envy of the world)
Working conditions, including sick pay, maternity pay
Very little corruption.
Great organic food at farmer's markets
Affordable food shop in Tesco, Asda etc
Restaurants from almost every culture
Affordable general shopping, and 24/7 opening hours.
Diverse and affordable high-street fashion
Great road network
Not-so-bad rail network
Fantastic travel opportunities for Europe and beyond. Many airports are a global hub. Affordable flights.
Everybody speaks English.
The climate (yes, really - I think it's perfect. Not too extreme, a rotating mixture of every weather type)
Phew! A long list, but I quite enjoyed writing it. I'm so lucky I can go back and live in the UK whenever I please, unlike the millions of people the world round who can only dream of it.