Your crystal ball? Our national interests are not that huge nowadays, WEBF. The number of possible threat nations where HNS would not be available (but where a carrier would still be able to operate without nearby shore based support) is a VERY short list. The kind of contingency that necesitates using a carrier is very very rare and very very unlikely, yet you want the UK to spend billions on preparing for such a contingency. Expecting you to justify such disproportionate expense is not unreasonable.
ORAC makes a sensible, reasonable point, and trying to wriggle out of making a proper answer by being flippant is below par.
"What if our national interests are threatened but no other nation is directly involved?"
Call in favours from allies. If that doesn't work then use Tomahawk. Use Storm Shadow from MRA4. Use GR4/Harrier GR7 from an adjacent nation.
If none of that works, then accept (AS YOUR GOVERNMENT HAS) that we cannot do everything.
"We really can't expect the US to hold our hand every time."
We probably can, you know. And if not them then our EU partners. And if not, then accept (AS YOUR GOVERNMENT HAS) that we cannot do everything.