I couldn't find anything during a half-hour peruse through the rule book, but, actually, I could never approve of, say, Labour declining to field a candidate in my constituency (historically a battleground between LibDems and Tories with Labour always a distant third). I want it to be up to me whether I vote tactically or not. I did just that in 2010 in order to keep the Tory out ("Don't let the Tories win here" was the campaign slogan) which worked, but then the "progressive" LibDems sold their soul to the devil and propped up a party that went on to subject the poorest in the country to years of misery. As far as I know, MPs can be whipped or not to support a motion from another party, but I'm not up for formal deals to cooperate. We have a party system in which the parties each have their own set of values. As far as I'm concerned, there's only one other significant "progressive" party and that's the SNP, but their values and aspirations are very different to Labour's in many regards. And, not least, the SNP hegemony in Scotland is one of the main factors keeping Labour out of power. Which, of course, is Labour's problem.