In all fairness, in the UK, but it's a little outdated (from the perspective of younger speakers), who are more likely to use. 'to be fair'. Some people use both, and some even think there is a subtly different way of using them. Possibly the younger speakers over analysis the grammar and think "how can you be in/inside fairness". Because their English education does not cover the essential groundwork of English it's ancient roots which still form the bedrock of all our daily conversations. EDIT yes it is real English.

Hello, Tiger. I hear people say, "To be fair" much more often than "In fairness". It has the same meaning but is more updated in how it's worded. "In fairness" sounds a bit out of date and formal for everyday use, in my opinion. But "to be fair" is very commonly used, in everyday conversation and on TV shows and movies as well.

Used in Australia as well.

Yes, “in (all) fairness” is used (in the US).

Hi! Having stayed in the Philippines, I know it can be confusing to know if their expressions are just "Filipino-isms" or more widely used in English! Yes, in New Zealand at least (therefore likely the UK and Australia as well) I often hear "in fairness" or "in all fairness" - it's just not as common as you say it is over there. In the Philippines I started saying "already" way too much because they say it so much there, even when it's unnecessary! Hope this helps :)