What "fleet" are they going to escort?
You could put a bet on us actually floating a CVF that's fit to put in harm's way, but you'd be better off gambling on England winning the world cup, or giving your cash to a gypsy in return for some Magic beans.
By the time the CVFs are supposed to be in service (which they won't be), the Lusty and Ark are going to be held together by their paint.
I suppose at some point a grown up might get involved, buy some Malaysian freighters and weld a big helicopter flight deck on top. Then we'd have ships with equivalent combat performance to the Invincibles - which are basically helicopter carriers now as the GR7s start falling apart - superior performance to the type 45 (or, God help us, type 26), and at a fraction of the price of any of them.
But then, why deploy a billion pound ship to defend a much cheaper one, that can strike back harder? Especially when it can't defend it anyway?
Bah, these ships are just wank fantasy material for recruiting ads and salty seaman still re-fighting the Falklands, and welfare for BAE.