Tony, to confirm what Cindy has said, I've just returned from a holiday, the JY rep at Heathrow attempted to book a few us in online at the same time (as we'd all arrived around the same time), it was a bit messy as the system began to time-out on her (and she "blamed" us for dithering!), most of us had our own request, for e.g. a window or aisle seat respectively, so I'd just say to you, you can book your seat yourself when you get to the airport, either via the new check-in booths, or at the counter when you check in your baggage, if you get there early enough, fingers crossed you'll get the seat you want.
