If this is achieved without afterburners (as I understand it is), then yes, this is an immense difference in fuel consumption and radically impacts the economics.
Great Britain is the name of the post 1707 Union of England and Scotland. It's not neutral if you are a member of the Unionist community in Northern Ireland.
The United Kingdom is the name used by the country at the UN but is, of course, not neutral if one is a member of the Nationalist community in Northern Ireland.
Here .... en-ie and en-gb are different things, because the Irish English dialect is a bit more distant from British English than the Scottish one. Whereas .uk is a country code representing a specific political entity.
Aside from any such associations, they mean different things.
People often use the terms imprecisely, but (for example) if Scotland were to leave the United Kingdom, it'd still be part of the island of Great Britain.
Afterburner is a common term in both countries. Reheat is and always was a semi-official slang used in the UK and is a reference to the similarity in concept to reheat steam turbine.