The legal boundary that will separate Britain from the EU. Describe the geographical contents that will be within that boundary after Brexit.
It’s ‘Britain’, a geographical rather than administrative/political/constitutional term, which is confusing. The border is that of the United Kingdom. It consists of a sea border between England/Wales/Scotland and all other states, plus a land border between Northern Ireland and the Republic of Ireland. Great Britain (England, Wales and Scotland), together with Northern Ireland, form the United Kingdom.