No 6 is in Dublin, which although within the British Isles, is not in Britain, but Ireland.
The specific meaning of the word "Britain" is a bit blurred, but generally defined as as England, Scotland and Wales, the same as Great Britain.
Great Britain however, is a geographical term to define the largest of the British isles.
Northern Ireland, although part of the United Kingdom is not part of Britain as it is generally defined, but a citizen of Northern Ireland is officially British.
In all the blurring of the terms Britain, Great Britain, Great Britain and Northern Ireland, and United Kingdom, there is no possible way that a place in the Republic of Ireland could be said to be in Britain.