Once you've figured out which carrier you want to fly on, and which has seats available that is when you should move miles around. Both DL and CO allow "instant" redemption from MR points so the transfer just happens, and CO will hold the seats in case you need to call to move the points or something like that.

My recommendation would be to fly the DL/ExpressJet flight from PHX to LAX and then use either VS or AF to LHR from there. That will get you the best value for your points and the most comfortable long-haul flight, assuming the seats are available. CO can redeem on both VS and AF, so that should help you out there. And I'm not sure exact dates that you're looking for, but CO has at least one seat on the VS flight on the 23rd of December.