I was able to choose seats at the time of on-line booking; no additional fare (economy advanced purchase).
Seats could also be updated after on-line ticketing, but web pages were particularly small, tortuous, and illogical.
On-line check-in U.K. worked as advertised, seats and service good (newer A330-300 Boston).
More difficult from within US (no printer), but new self check-in systems appearing in the airport; again no problems.
Good route, good service, tolerable delays outbound (within EU limits), Dublin, and US pre-clearance to be recommended, even with several simultaneous US departures.