Well by analogy to syllogisms in my recent post: What are France, the UK, Germany and Italy doing in the G8?, and since the USA has 1 seat not 50+1 in the UN, the EU27 should have only 1 seat not 27+1, in the UN Plenary and its Security Council!
The idea of the EU (and its High Rep) using ad-hoc the UK's or France's seat in the Security Council is cause for making the EU and its members a laughing stock! The EU and its 27 members should be represented by a single seat as pointed above.
The same applies, IMO, in the case of the WTO. The current situation in the G8, the WTO and other international bodies is, to say the least. absurd and this is one area that the BRICs should be making waves about, not whether a EUropean becomes again the head of the IMF. Maybe it is the proper type of trade-off.
And what makes common sense!