Apologies if there've been threads on this that I've missed. As I understand it, they want a moderate centrist politics something along the lines of European social democrats, plus they're all pro-EU.
Isn't this roughly the position of the Lib Dems? And they have a well established party machine and are - allowing for the problems of 1st past the post - actually quite popular.
I should say I've never voted LD, but they do seem a fair match for what this group of MPs are saying they want. I don't even think it's careerism, as the LDs have taken in defectors in the past and promoted them fast 
Is this just going to be a re-run of the 80s with them going alone for a while a la the Gang of Four then folding into the LDs in the end??