It all depends, as usual, on the definition of homeless. My wife is a Housing Advice Officer for the local council, and the way they are forced to bend the definitions is appalling. There are definitions for homeless, roofless, unintentionally or intentionally homeless, temporarily or permanent. There are also problems with the link between under 18s (who are officially and legally children) and the way that social services in this region do not like taking them on to their books. If they are in a hostel, or the staff believe that they are not truly rough sleeping, then that skews the numbers as well. There is no way that the figures are that low, it's stretching credibility to believe otherwise.
If you can find a government official who will venture out at any time of night to the areas that these people are living, unless they are accompanied by the police and are on at least double time, then good luck. The ones who have to deal with these people on a daily basis certainly don't have company cars, overtime rates, or the luxury of going golfing with the local Chief of Police.
I buy the Big Issue from authorised sellers, and I also quite happily buy sandwiches and drinks, but never part with cash. There are some sad, sad stories out there, the homeless are not always the workshy and the feckless. They reckon that almost everyone is only two mortgage payments from joining them on the streets.