i dont believe it was a "guillotine" motion as debate had only just started, it was a strict programme to prevent Tory rebels talking the bill to death. such was the technicalities and procedures that mired the whole issue. Cameron couldnt get enough Tory votes for the bill as presented but was supporting it. bottom line is Labour block progress of the bill which they supported, presumably to create trouble for the coalition. why insist on more debating time when in agreement?
As I said earlier, the Labour party supported it - they didn't block its progress at all. "The rebellion by the 91 Tory MPs failed to block the second reading of the bill which was passed after Ed Miliband instructed his MPs to give their backing. The bill was given a second reading by 462 votes to 124, a majority of 338, in one of the largest votes of the parliament"
http://www.theguardian.com/politics/2012/jul/10/house-of-lords-reform-halted
The sole reason the bill failed was the size of the Tory rebellion. If the Tories had negotiated with the rebels and pushed through a compromise, the bill would have sailed through - it was a manifesto commitment by all three parties. But because Cameron is not a strong enough leader, he back out rather than deal with his own MPs