I think the rule of thumb Vallance applied then was current deaths multiplied by 1000, per Neil Ferguson extrapolated from estimated CFR and growth rate.
Cool - means (taking this work at face value) it was a good rule of thumb.
They note in the paper that age of population is currently not being taken into account, but they aim to add it. That will be a fuck load of busywork, but it will be really interesting to see the impact it has.
Just to add, this roughly matches the under reporting estimates given last week by Vallance (5,000-10,000 cases with 500 reported).