Sorry to be a pedant, but it is the UK ambassador to the UN who is actually "on the Security Council".
(regardless of PM)
I assume everyone on this forum knows this, so indeed a very pedantic comment. It was shorthand for "an ambassador following the orders of ...". If we're going to have to write as literally as you demand, we will need a lot more words on this forum and a lot less clear writing. Next thing you know you cannot write a PM is in "the driver's seat", because actually he's not in a car but in the PM office!
