As a general rule, you will probably get better prices with LD Lines when booking well in advance. The opposite often applies with Brittany Ferries, as I often hear of people checking fares after booking and finding prices are slightly lower. If BF has spare capacity the company will sometimes reduce fares to fill the space; the system is driven by expected demand and actual supply. Bear in mind that with all ferries except the small high speed ones it's the freight traffic that provides the bulk of the income. You and I are either a bonus or an inconvenience, depending on company policy. That's why you don't get the same 'seat sales' offered by budget airlines. You did in the days of duty free sales, but that was a different situation altogether.
LD Lines tends to follow its terms and conditions to the letter as far as amendments are concerned - but can sometimes be sympathetic if you speak to the right person.
I have had some very good fares with BF for crossings booked at the last minute, and have also made changes to bookings; even getting a refund when the new voyage is in a lower price band than the old.
With the cost of oil at its current high level - and environmental regulations in force in the Channel, North Sea and Baltic dictating the use of low-sulphur fuels rather than the cheaper bunker fuel permitted in mid-Ocean - ferry prices are highly unlikely to fall.
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