If you don't take any leg, then don't bother showing up for later segments - they will have been cancelled!

if you really can't make the first flight of the return leg, you need to get on the phone to the ticketing agent and make the necessary changes to the ticket, or you risk losing the entirety of the return. Or just book it as an open-jaw (LAX-LHR-MAN, LHR-LAX) to start with.