Delay could become an election liability itself
The lack of a referendum is already a contentious issue and it will become an election issue if Brown calls an election shortly after he has ratified the Lisbon Treaty—adding weight to the claim that the government doesn't consult/listen. This would make winning the election less likely.
From a comment by Andrew McEwan:
"Post-election ratification would give a government a greater mandate
Andrew McEwan (6/22/2009 3:50:00 PM GMT)
Labour endanger the election being about government's ability to listen rather than policy if they delay the election until after the Lisbon Treaty is ratified. The lack of a referendum is already a contentious issue and it will become an election issue if Brown calls an election shortly after he has ratified the Lisbon Treaty. This would make winning the election less likely."