The Prime Ministers flights are usually arranged for Number 10 by a Gatwick based broker, you know the ones who want to be your partner in the air. They're always happy to tell you how good they are at things like that, even going to great lengths to tell you about their Royal Warrant. However when things go wrong (like when Mr Blair's DC-8 went pop in South Africa a while back) they go strangely quiet. Don't they guarantee a replacement aircraft if yours goes tech? Hmmmm.
Basically, Number 10 are asking the broker to find best value aircraft available at the time to perform the flight, which is always going to be cheaper than the Prime Minister having his own aircraft on standby "just in case". Personally, I'd rather this was how my taxes were being spent.