Liberals at CBS tried that argument with the forged military document debacle. "It has to be true since the Pentagon didn't deny it" was something they were quite fond of repeating over and over again.
This is a completely different situation. The Rather documents were old and nobody in the Pentagon could be sure whether or not they were real, so the government couldn't deny it because they couldn't be sure.
The DSM is less than three years old and lists specific people supposed to be saying specific things. All the British government would have to do is ask them, "Did you say such and such on such and such day?" If they didn't, then it's a fake and the Brits would have said so. The fact that they haven't denied it means they know those things were said.