This is what I did last year.
I phoned around April to enquire about a December crossing. I was advised that the Winter sailings weren't yet confirmed, but the timetable would be released about 1st July (if I recall correctly).
I was offered the option to pre-book which I chose to do. All that Brittany wanted was my preferred route and date and they would book as close to my requirements as possible once the timetable was confirmed. In due course I received an email with the booking details and confirmation of the booking deposit having been paid.
It worked well ...
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