Primus inter pares
or the First Among Equals
A position that has had some fine inhabitants over the years, such as Churchill, Thatcher, Disraeli & Pitt.
Also some utter wnakers, chief among them the current incumbent (see Tony Bliar), his likely successor, that clown Major and any amount of insignificant losers.
I realise this isn't terribly incisive political analysis and comment, but I really just hate TCB.