It is a myth!
The country is the United Kingdom, UK for short. The UK is by statute The United Kingdom of Great Britain and Northern Ireland. Check in your passport if you don't believe me. Great Britain is correctly described above. NB there is no mention of Mainland UK at all. It is ficticious, it does not exist, it isn't, it if it wasn't nailed to the perch it would fall off! It is a connivance concocted by those too lazy to know any better, in particular the various courier companies who feel they need to surcharge carriage to Northern Ireland and have not the whit to describe it correctly. One would expect an part the UK government to know what constitutes the country and to correctly use the statutary description. I think there are specific laws about describing it incorrectly but these seem not to matter any more.
Now I understand what they mean by it but why must they invent a new name for it. Is it not a spade, why call it a shovel! It is Great Britain, NOT Mainland UK.
Anyway managed to register my acquisition this morning, curious, in spite of it being disowned by Swansea, it is not considered a first registration in Northern Ireland.