You only need to avoid U, P and T in "GoLight". All Flex fares are upgradeable. The booking class is actually very easy to find when making a booking on the A3 website. Once you've selected the flights you want, click the down arrow next to "FLIGHTS" on the right hand side of the screen and you will be presented with the booking class. If you want to force a flight into a higher booking class, the only option is to ring A3 to book. There's a €20 fee for offline bookings.
There's lots of really good info on booking classes and upgrade coupons in the
Seat upgrade coupons thread.
Hope that helps and that you're able to get your segments done in style.
