The better question....when did the RN last have a credible carrier?
In case people have got a bit dumber lately; we only need 2 carriers because there's always a lead-in time to a conflict/crisis.
Such as the FI squabble....and if one of the carriers is in drydock undergoing overhaul....and the other is on the wrong side of the world with some sort of committment?
The US is down to 12 from 15 large carriers....and they become in short supply often enough as it is.