Hi Paul
You will get your flight details with the paperwork and check in opens 60 days before. You will already have a seat pre-selected, however if you would like to change you can, depending on the seat you want to move to there may be a change (i.e emergency exit row).
In terms of the new baggage policy for your booking it states: you can bring one small cabin bag which must fit under the seat in front of you (40cm x 20cm x 25cm). Priority customers (which you are) can also bring an additional 10kg bag on board which must fit into the baggage sizer (not exceeding 10kg and dimensions 55cm x 40cm x 20cm) and will be stored in the overhead locker.
Hope that helps.
Kind regards
Matt