If you are using BA, they fly 747 -400's and there are no seats with emergency exits; they are all by galleys etc. If you can afford it, I highly recommend going up to World Traveller Plus (assuming you are in World Traveller at the moment). I did this in April and it makes a hell of a difference. Also, try and get a 'bulkhead' seat as these usually have more legroom and you definately don't have anyone reclining their seat into your face!! Heres a seating plan. http://www.seatguru.com/airlines/British_Airwa ys/British_Airways_Boeing_747-400_A.php
I find bulkhead seats a mixed blessing: you may find you have more room in front of your head (as no seats in front tilting back) but actually less for your feet (as no seats in front to put your feet under). It depends how tall you are, but I find BA seats in economy to be more cramped than most.
As for checking in online, you just have to be quick to get the seats you want..
BA opens up their checkin for all 24hrs before the plane leaves. So make sure you log on then to get the widest selection of seats. If you have to travel economy I would go for the front seats - no one putting their seat back into your lap.
Assuming British air regulations to be similar to our FAA rules here in the U.S., you cannot book an emergency exit row seat until check in since they airline has verify that you meet the requirements, such as minimum age, health and other factors for opening the emergency exit if needed...