We don't know that Omicron is mild. South Africa has a much younger population then the UK. There isn't enough UK data yet.

To talk in respect of risk to individuals is to miss the point. We do know that Omicron is far more transmissible than earlier strains. That means that a lot of people are going to get it over a short period of time. Even if only a small proportion develop serious illness that could risk overwhelming the ability of the NHS to provide care (bearing in mind that a lot of NHS staff could be affected).