The explanation

Don't mock!  The Telegraph may not be far from the truth:

“So what was it?” demanded Sam. “What made you use the veto? Was is the fear of having to hold a referendum and splitting the fragile Coalition?”
Dave lowered his voice. “Do you remember what happened to Chamberlain when he went weak on Europe?”
“The Second World War?”
“Worse. His long-term rival from the Right of the party took over.”
“Oh, I see,” said Sam. “Boris was right: you have played a blinder.” 

 Or perhaps not ...