I thought of :
- The Viking Invasions
- The Norman Conquest
- Stephen v Matilda
- The Wars of the Roses
- The Spanish Armada (but then dismissed that as a scary thing that basically just got blown away)
- The English Civil War
- 1916 (although if you argued that this is a lot more than England's, Britain's, or even the UK's Hour, I'd have to concede the point)
Then it occurred to me that we actually have a gadget that is supposed to specifically indicate Hours of Need just down the road at Buckland Abbey, so I looked up Drake's Drum to see what times of national emergency it had seen fit to signal. But it seems to be a most erratic indicator, drumming for things like Lord Nelson being given the Freedom of Plymouth, which doesn't really seem like an emergency, even in Plymouth.
Incidentally, there's an excellent list on Wikipedia of Sleeping Kings** In Mountains. I knew there were quite a few of them, but I hadn't previously realised quite what a superb range of sleeping heroes was available in the event of emergency.
* I'm not being too picky about national definitions here, although I think 'Albion's Hour of Greatest Need' definitely has more of a ring to it than 'United Kingdom Maximum Necessity Moment' or similar.
**Although not all of them are kings.