If Gibraltar is counted the UK should not be considered an island country, so Ireland and Cyprus would be the answers by bordering UK whose territory is on a continent.
I've never seen Cyprus treated as bordering UK on this website, but if it was the case, shouldn't we consider USA as bordering Cuba because of Guantanamo Bay?
Actually, military bases would gave the USA common borders with probably a dozen European and Asian countries...
Akrotiri and Dhekelia is not only a military base, but a british overseas territory, which include a military base, that UK keept when Cyprus got independence.
There is a land border on Passport Island between Bahrain and Saudi Arabia. For Singapore the border would be up in the air on a bridge. Check out the caveats.
Actually, military bases would gave the USA common borders with probably a dozen European and Asian countries...
If you’ve mastered this quiz you might want to try this one:
https://www.jetpunk.com/user-quizzes/1306568/countries-bordering-island-countries