How often is seat E go empty in Y class on a 777?

I am flying from LAX - LHR (#136/137) in a couple of weeks in Y on the 777-200. I bought the ticket when they were $115 each way and am in Y class. The exit rows and any good seats are already taken. Right now I have H & J in the mini cabin. I was wondering if I should try for C & D or F & G in hopes that E will go empty. I am flying to LHR on a Wednesday night and returing on a Monday. Right now

I will also try again (or hope) at the airport to get the exit rows at 31 but it is unlikely.

Thanks.