Call and ask the airline how full a flight is and don't book over the phone if they have a charge but get an idea of how flights are. I have worked the gate on a lot of flights to London and I often suggest that you get a seat in one of the last 4 rows in the middle bank of seats, because those are the seats that fill up last. A middle seat in row 33, 34, or 35 on an aircraft that has 35 rows will often be open - even when a flight is relatively full. Especially nab it if you can see that the other aisle is already reserved.
The bulkhead can be good but sometimes it isn't that great. You have to store everything above you most of the time, during turbulence, take-off, landing. Plus neighboring passengers will be jostling for that same overhead bin space...yuck! Also the back is often better especially if there is a galley at the back. You can have bottles/food reheated quickly, usually there is a lav with a change table, etc.