I fear we are now not far off being in that very boat, made even worse by a risk aversion and political correctness that hampers our abilities even further. My old history teacher used to tell us that in the event of WW3, we might last a week if we were lucky and were effectively just a stalling device until the US could mobilise. That was when we had several times the size of forces we have now. How long do you think we would last these days?
I don't know what the answer is but part of it ought to be in educating the general public as to the true state of the military. I know it's the standard assumption that the populace at large cares only about health, social services and education but that's partly because politicians only focus on those, admittedly important, issues. Most people probably believe the BS that because we have the "fifth largest military expenditure in the world" or whatever, we have a truly fit for for purpose military. If they really understood how limited and fragile is our capability vs emerging threats, they may actually actively support greater expenditure. But as this would just make life uncomfortable for politicians, they aren't inclined to support such an education of the public and instead continue to make hollow announcements about 2% etc. The real issue is that it is inevitable, at some point, that our armed forces will once again be forced into a situation for which they are totally under-equipped and lives will be lost as a result. Which politician will put their hand up and take the blame for that I wonder?