Shirley,
The F-35 B is a lot more versatile, though an advanced Harrier ( as now with Sniper ) would seem quite up to the job ?
If the UK can suddenly come up with £400 Billion to save bankers - a rhyming slang if ever I heard one - what's the snag with £ 4 Billion for a couple of carriers, + the F-35B could use assault ships ?
As for shooting asteroids a la Hollywood, isn't that asking a bit much of any aircraft including the supposedly banned F-15 ASAT, while there are other systems which could ( hopefully ) do the job !