Years ago someone told me it was a phrase made popular by Maggie Thatcher during the Falklands conflict concerning the huge overtime bill BBC film crews were clocking up.
Lew Grade, an entertainment mogul, famously used this expression to describe getting the rights to broadcast the first commercial TV in Britain. Getting the rights for his company , which was Granada TV, was , he said, 'A licence to print money'.
I first heard it shortly after the setting up of STV - the Scottish television commercial channel. It was a remark made by the then chairman, Roy Thompson, about the amount of money commercial TV could make. His remark was widely reported. When the company went public, its value was reckoned to be 22 times the cost of the original investment. This was back in the 1960s, well before Maggie Thatcher came along.
Yes, Heathfield, thinking about it, I recall it first used by Thompson. (Googling gave it for Lew Grade, who might have used it too). Thompson (Lord Thompson of Fleet) owned newspapers; the Daily Express being one; and was thinking of his printing newspapers, which was pretty profitable, compared to the licences to run commercial TV stations.
Actually, (Lord) Lew Grade helped start ATV (Associated TeleVision) not Granada, back in 1955. Granada was started by (Lord) Sidney Bernstein and they were bitter rivals. It is correct to say that Lord Thompson used the expression when setting up STV though!