Britain has recently begun a campaign to expel Russian diplomats from Europe. But let's say tomorrow new evidence shows up proving that Russia wasn't involved - the UK would certainly be in a bad position and might be forced to apologize if they ever admit they are wrong.
But has there ever been a precedent where a state apologized to another state after going as far as expelling their diplomats? Or should one expect the UK to 'maintain face' no matter what as its not common practice in diplomacy to apologize? Note that I'm only interested in cases of voluntary apologies, not situations where someone does it after losing a war or at the threat of an invasion.