Relative economic and military decline of a medium sized nation is inevitable, it has little to do with immigration. How can a country of 60 millions maintain world supremacy with a world population of more than 100 times that number.
Today there is greater access to education for the masses and a relatively (but still not totally) free market. As the latter factors dominate, India and China will become the powerhouses of the world, and the US could find itself slipping behind them as well. The UK is hardly poor anyway, no country has a significantly higher GDP/capita except the US and some very small states (Japan and Canada only slightly more).
Regarding immigration, some believe it is they who keep the UK and US economies afloat doing a greater proportion of real work. So many people nowadays seem to be managers or glorified administrators of some sort.
