1) We're never going to do a major op without the Yanks. Like it or not, that's the decision.
2) For that reason we don't need to do everything. They will always be there.
3) We haven't had to use our carriers (that's HAD TO) since 1982. Even if we were going to do autonomous stuff, a carrier wouldn't always (or even often) be required.
4) A UK Carrier has never been top of the US shopping list of assets to request in coalition ops. They often want our recce, Elint, tankers, etc., and they'd want our SEAD if we had any. If we want influence and power, we need to have the capabilities that our potential coalition wants. And they have enough carriers of their own.... and access to French, Spanish, Italian, etc.
5) The assets they want are also those that we would have to use EVERY time if we did do anything autonomously.
6) Therefore spending money on carriers seems a poor choice when money is so tight.