I think your answer is pretty simple:
In 1980, the average house price in the UK was £23,287 - so to be a millionaire, you had to have £976k liquid assets. Adjusted for inflation, those £976k would be £4,463,578 today. That is still a lot of cash/liquid assets.
However, the average house price is £231k today, with London houses closer to £600k. Doesn't take so much liquid assets to make a millionaire on top of that, less than a tenth of the buying power compared to 1980.