Is there anywhere that has not been at war with Britain?
"Anywhere" would mean "a contemporary country that was part of an empire or state that was at war with the United Kingdom or one of the earlier constituent countries of the British Isles". For "was part of an empire or state" we can say "was claimed by and/or under the control of an empire or state at war with Britain, and someone from there could have conceivably met the British in battle other than as a simple mercenary." Add: Private adventurers and mercenaries should be excluded.
So all of South America counts, as Britain was at war with the Spanish Empire several times. Switzerland counts, as it fought at some times on the French side in the Napoleonic wars. China counts (opium wars). Norway counts, several medieval wars with England and Scotland. Turkmenistan and Kyrgyzstan count, as there was an Anglo-Indian army fighting there in 1918-9.
Perhaps Mongolia? Anywhere else?