After WW1 the UK was still the leading power politically. After WW2 it most defiantly was not. WW1 had set the stage for that decline to happen but it hadn't quite happened yet.
It should be noted many commenters ain the twenties and thirties thought war between the USA and UK, as the new took over from the old, was highly likely. WW2 prevented that being necessary as it shifted power from the old to the new.
But, coming back to the Poland argument, Britain was only in that position because everyone else was severely weakened, it was not based on Britain being stronger. Britain was just less weak than everyone else in 1920. When those countries rebuilt and started to look beyond internal affairs the British position was exposed and the British policy makers were well aware of that fact.
Publicly, the British are living in something of a fool's paradise in the 20's. Politically there is a dim awareness that 'it cannot last forever', by the thirties it was really obvious. So it was not WW2 that altered Britain's position, just the fact that everyone else got over the WW1 shock and started to look at, in some cases, restructuring the global system that had underpinned British (and French) strength.
K