There's a book out called "All the Countries We've Ever Invaded (And the Few We Never Got Round To)" by Stuart Laycock which explains this on a nation-by-nation basis.

The countries Britain has yet to invade:

Andorra, Belarus, Bolivia, Burundi, Central African Republic, Chad, (Republic of) Congo, Guatemala, Ivory Coast, Liechtenstein, Luxembourg, Kyrgyzstan, Mali, Marshall Islands, Monaco, Mongolia, Paraguay, Sao Tome + Principes, Sweden, Tajikistan, Uzbekistan, The Vatican,

The two principal reasons these countries haven't been invaded (yet) are that they are landlocked (so the British navy couldn't reach them), and that they are too small and insignificant for the British to invade.

Sweden stands as an excellent exception to this.