"I'm sorry, but if I see somebody starting to source information from such tabloids you mentioned, especially information on biographies of living people regarding stuff that is not confirmed, there are going to be problems with me."-MuZemike
All well in theory, but have you looked? The Daily Mail, Sun and various other tabloids are regularly used as sources on BLPs. The typical way of getting round the reliability issue will be to use phrases likes "it was reported in the popular press that...", on the pretext that that anything tabloids report is notable by virtue of being reported in popular newspapers (regardless of whether the source is reliable or not wrt the facts). After all: "surely that The Sun has said x is notable, and The Sun is a reliable source regarding what The Sun has said." :(
As has been said, Wikipedia has yet to define what it means by "reliable source", and "notable source" is very easily substituted as a metric, with the small safeguard of attribution (sometimes).
Scott
One is expected to use sound editorial judgment. Using British tabloids for a biography of a living person falls outside that remit. One is expected to have some familiarity with what is an appropriate source for the subject.
Fred Bauder