In regards to it being mild, Chris Whitty addressed this during the press conference today.
It is partly "mild" because of the existing population immunity - such a large proportion has now been either vaccinated or had covid (or both) that it will be milder for them than it would be for someone unvaccinated who has never had covid. I have seen other scientists say the same on Twitter.
Equally though as it's spreading so much faster and more people will be ill at the same time, a small percentage of a large number is still a lot of people so that's why it can be more of a risk for hospital capacity and essential services in general.