I'd hazard a guess that they're based on the claimant count for jobseekers allowance and therefore discount those on income support, single parent benefits, incapacity benefit, refugee benefits, asylum seeking benefits etc etc.
Incapacity benefit alone has more recipients than jobseekers allowance.
In it's last report on the subject, the Office for National Statistics stated that one third of UK households derive most of their income from welfare benefits. That fits in quite nicely with the fact that the government makes about 22 million benefit payments per month and it spends around a third of its income on welfare. That's more than on health and education combined.
True unemployment levels, measured using internationally agreed techniques, are thought to be about treble the official levels spun by the government. That would put us right at the top of the unemployment league table.