My understanding is that this figure was calculated on the basis of what could have happened before the UK government changed it's policy to "stay at home". Initially the policy was to "allow" healthy people to simply catch the virus in the hope of obtaining "herd immunity" but then it was realised that if this policy was pursued hospitals would likely be inundated with shed loads of casualties and so government policy "evolved".
Although the UK now has the "stay at home" policy many people are still expected to work. This means that unless you are able to work from home or are able to grow very long arms, you may still have to go into work. The UK is slightly different from many other countries in that it hasn't stopped all "non essential work". This could still happen, of-course, if the UK government deemed it desirable in order to control the virus.