I think they will radically restructure themselves to follow proper socialist ideals, work hard to develop a popular following and with mass support throw out the Tories and renationalise everything.
Only joking! They'll bugger about for a while voting in ridiculous party leaders and then kicking them out, picking up Tory policies or whatever seems to be in the papers a lot, stabbing each other in the back, whinging about losing the election, blaming each other for it and eulogising Blair (maybe).