4 ... the number of aircraft carriers the UK actually needs to make it worthwhile IMHO. (plus the aircraft to operate from them and the personnel to man them). If you don't pay the full insurance premium there's a shortfall when you claim.
I was reminded of a scene from the West Wing:
DONNA
$500 screwdrivers is why you didn't vote for the President?
JACK
I work for the President. That's a lot.
DONNA
It's wasteful spending.
JACK
No, it's not.
DONNA
A $400 ashtray?
Jack picks up a wrench and smashes an ashtray that's on his desk. It breaks into three large chunks.
DONNA
What was that?
JACK
A $400 ashtray. It's off the U.S.S. Greenville, a nuclear attack submarine and a likely target for a torpedo. When you get hit with one, you've got enough problems without glass flying into the eyes of the navigator and the Officer of the Deck. This one's built to break into three dull pieces. We lead a slightly different life out there and it costs a little more money.