The first thing to do is decide where (and I mean what country) you want to live in. Bear in mind tax regimes.
The second thing to do is to get top grade tax and investment advice, and to think about the balance between income and capital gain.
The third thing to do is .... pretty much anything you like, with a few exceptions.
Most people here are talking about interest rates. If you have that kind of money, interest rates are generally not what you worry about. What you worry about is the capital gain, because the options for increasing that money through careful capital gain FAR exceed any standard interest rate. But anything you do will involve an element of risk, so a careful spread of investments is mandatory, if you have any sense. So look at gilts (probably low yield ones), look at property (including some speculative markets), look at some high risk high return investments as well as some stolid, blue chip ones, and so on.