In English yes, you would say on. However, I think for the purposes of this exercise the contributors are correct. Gaelic says you are "in" a place rather than "on" it.
I mean if you think about it, the English doesn't have much consistency anyway. I am currently "in" Britain, even though Britain is an island which I am standing "on".
The Gaelic doesn't change whether it's ann am Muile, ann an Alba, or ann am Breatain.
According of the Wiki article (I'm not taking a position on its accuracy), "Britain" means the United Kingdom.
"Britain usually refers to the United Kingdom, a sovereign state comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands." https://en.wikipedia.org/wiki/Britain
But then Wiktionary says "Britain" can mean the island: "The island of Great Britain, consisting of England, Scotland and Wales." https://en.wiktionary.org/wiki/Britain
I would sound very strange to me to say "on Britain/Great Britain," I think because they seem more like political entities. It's all very confusing.
Well I am sorry but for once I am going to say I know better than Wikipedia. Whilst some people may say that, even people in Britain, I think it is one of those things that people say without thinking or without understanding. They may think that is what it means, but if you ask them specifically if it includes Northern Ireland, I am pretty sure most would say 'no' or 'don't know'. And of those who said 'yes' I guess a good proportion would say if asked that it contained 'Ireland', the Channel Islands and the Isle of Man. None of these, apart from part of Ireland, are even in the UK, so they would basically be showing they did not know what they were talking about.
Certainly many people in Scotland, Wales and Ireland would be deeply offended if you suggested Northern Ireland was in Britain. Some people in England might neither know nor care.