I made the same trip with my then 14 month old, going we flew during the day to Los Angeles but that was mainly because we had an onward flight, when going you have to remember that Los Angeles is 8 hours behind the UK, i wouldn't want to leave the UK early evening have a child sleep all the way and then land in Los Angeles at 10pm their time and then have a wide awake child! On the way home however we flew early evening from Los Angeles and got back to London mid morning
I'm making the return leg this summer (we're flying to New York) with my now 4yo and we'll leave Los Angeles at 2115 and will get back to London mid afternoon