Yeah I would have thought they'd be able to get a rough figure by working out how many missing person reports are duplicates, and counting unidentified people in hospitals - but that the final figure may well be higher (because of people with no-one to report them, including lodgers and others living there unofficially (for example my housing association told me I don't have to tell them if I have a partner, family member, or lodger move in, so long as I don't over-occupy or operate a business (ie airb&b) - I don't know if thats the case in social housing generally - and there'll always be people staying with friends and family temporarily, not to mention subletting and airb&b going on either without the housing management's knowledge or in privately owned flats - and more of this going on because of the government's appalling housing policies).
But it seems odd that some sort of estimate isn't being given.