Lancer,
I travelled EzyJet last year. This is how they managed seat allocation.
1. At Check In you get a sequence number on your boarding pass. The earlier you check in the lower the number.
2. At the gate they make boarding announcements for people to board in lots of 20 according to their sequence number (lower numbers first). All boarding passes were inspected mannually to check the number.
3. Once you are past the gate lounge it is every passenger for themselves and the race is on to secure the best seats.
4. The crew assess the suitability of the people who have chosen the exit rows prior to door close and changes are made if necessary.