British Airways - Boarding passes

If these are BA flights and you have the basic face, then you can select seats beginning EXACTLY 24 hours before the flight at the time that do online check-in. If both legs are BA then it should let you do this for both at that time. You would do the same for your return flight. You might need to book the return boarding passes separately, that happened to us in 2010, and the second one is based on 24 hours before LHR departure time.
You can always pay the premium for seat selection in advance.

When we flew on BA to Venice our flight departed from Gatwick, so we had to take a bus there from Heathrow. Watch out for that when you book.

Three of the 4 BA flights to Venice each day depart from Gatwick; the 9:40 am one departs from Heathrow. If you can't make that flight, be sure to leave enough time to do the transfer to Gatwick. As for the boarding passes, Larry gave you the details. You can either pay $30 per seat to select in advance, or choose your seats without extra cost when you log in to print your boarding passes.

On flights within Europe, you normally can't choose seats before check-in. That's the way it works here, and this is regardless of airline. Business Class passengers and passengers with status can usually pick seats at booking time, though.

If you want to get allocated seats prior to the online-check-in period of 24 hours prior to departure you can buy the seat on the BA website for €12 per seat. You can access your BA booking on BA.com with your BA booking reference and name...

Economy is difficult, ESP if you're not in the executive club. If you book business or first you can get your seat ok ... Go through BA themselves or a travel agent who can get you VERY specific seats. See "seat guru" if you do a lot of flying. I tend to either book specific seats on booking ( non-economy) or wait for week before when I get a " manage your booking" e-mail (may just be for executive club) or don't bother. If you don't bother and you're late you often get put in the middle in economy (regular flyers have their preferred seats stored on a database and if room default to these).
S

I recently bought two round trip tix on BA from Orlando to Venice and was shocked to discover picking seats on the transatlantic flights would cost $38 each, and $15 each on the European flights. That adds $106 to the price of each ticket. I sure wish I had known it in advance.

Celeste You only have to pay if you want to buy them in advance, right? If you do your seat selektion 24 hours prior to departure you don't have to pay, right?