I disagree that the British think that they have been an underwhelming at war and in general their performance in warfare. I believe the difference is that we've always shown a degree more humility than our neighbours both East and West.Indeed I'd suggest this is correct. On the one hand you have the popular idea that the British simply suck at land warfare, which (like most such myths) is generally unjustified. On the other, even the British themselves think this way. Even some in the officer corps seemed to consider amateurism and simply 'having a bash' the way to go.
Whilst Europeans have a tendency to be brash and forthright about their triumphs and glories, Brits in contrast take a more understated approach. Just compare the architectural and cultural styles of the Sun King, The Soviet Union, The Nazis, Bonapartist France vs even Britain at the height of our empire. Take a stroll through the streets and parks of any European city and you will quickly notice that a great many are named after greatly individuals native to that land. Eternally etched into the addresses and consciousnesses of those who dwell there.
Prima facie Brits will always be more modest regarding our successes and this can lead to some (as illustrated by this thread) to think that we've been weak at warfare. Dig deeper though and it becomes very apparent that at least within British consciousness that we always thought that we've had a steely resolve that has always pulled us through. Even the cited example of the Charge of the Light Brigade is remembered more for the remarkable bravery of the men on that fateful day. Consider "We merry few", 'Zulu', Richard Sharpe, the Battle of Britain, The SAS. If you ask most kids on the street who they think are the best forces in the world are, most will respond that we are.
We haven't lost a major war since 1783. You would struggle to argue that Britain has underperformed militarily given our sterling record.