FWIW - I flew LAX - LHR on a year old 777-300ER in MCE. We were in 18AB. The lav wasn't really bothersome, though it was an overnight flight so most folks were sleeping, or trying to sleep.
On the 788, I'd take a seat in 8. MUCH more legroom and easier for the person in A or L to get up without overly disturbing the rest of the row.