As was most of the western world, recovering from the oil crisis of 73, and experiencing chaos as a result through the rest of the decade. Hardly unique to the UK.
UK was largest recipient of Marshall aid for having the most need along with France only slightly behind. Being used as you say for maintaining world power status, and into the general treasury fund. We spent less on infrastructure during Marshall aid than without! They pissed that away just like was done with all oil revenues.
Considering the vast majority of oil revenues went on empty tax cuts and benefit spending through Tory created recessions, I'm not sure we'd have noticed the difference. 1 or 2p off income tax barely registers. I know I noticed none of Lawson's tiny bribes. At all. :p
The politicians would not, and should not be managing it being better served as some sort of apolitical hands off trust. I don't think politicians manage Norway's fund. Now there is little doubt that it would not have survived the privatisation and asset stripping of the eighties on unscathed, but, like the NHS there's no reason to suppose it would have been destroyed. The Tories debated but chose not to re-privatise the NHS in 51. Thatcher had a selection of industries that were felt were beyond the pale that were subsequently privatised by later administrations. Created in 75 might have been a bit late for achieving adequate perception in the public consciousness so who knows.
Still, it remains one of the very many great missed British opportunities that could easily have been a couple of trillion or more by now. 15 years of additional contributions and growth. There's many nations have sovereign wealth funds now, we easily could have too.