I don't know but it seems to be based on if we kept things exactly as they are and do nothing more. Maybe most other countries are doing more, or they are also on course for a similar worst case scenario if they don't implement additional measures.
If the government are saying they are 'following the science', then I can absolutely understand why the scientists might want to come out and say if they are actually advising differently.
More generally, I'm certainly not a fan of another lockdown - but given where we are right now, with cases increasing, hospitalisations rising and in some areas approaching capacity, with doctors saying they fear the NHS being overwhelmed, with other surgeries already having to be postponed due to those increased hospital cases - what is the alternative to a lock-down, right now? Not what the government could have done differently, or should have done differently, but given where we are now, is there an alternative, other than letting things ride, risking the NHS getting over-burdened and not being able to deal with anything else, and letting people die (not all of whom would be otherwise on death's door)?