The aforementioned increase in deposit and resulting reduction in speculative double bookings is bound to have made an impact for a start - 4% isn’t a big enough number that it couldn’t conceivably not be a result of such a policy. There’s also the fact that BF’s numbers are already in decline on some routes with the Caen route down by 7.4% in 2017 vs 2016 for example (by BF’s own admission). Portsmouth to Cherbourg was down 9% over summer 2018 according to BF’s latest published figures yet other routes were slightly up. I don’t see DFDS or P&O complaining to the press about how brexit is killing their business, nor Eurotunnel for that matter!!! While there may be a bit of a brexit effect I personally feel it’s more likely people are using different travel options (airlines, Dover Strait, Holiday elsewhere than France) and not double booking anymore due to the financial penalty
With the introduction of the deposit etc maybe the drop in passengers is only reflecting the true bookings and not an inflated figure due to speculative bookings previously experienced. Judgement is probably best left until this time next year.
With Brexit there is also the possibility, as Stenaline have stated, of relocating ships to other more direct routes from Ireland to France and Spain.
Important to note Stena have said it’s a possibility but that it’s not something they intend to do at present.
P&O are reporting they’ve had a really strong summer and a good 2018 so far with record breaking passenger numbers on the Calais route in August, while Brittany Ferries appear to be down. Of course that’s this year not next, but it seems Brittany Ferries are losing business to the competition which could also help explain why forward bookings are down. Or people may genuinely be being more cautious - it’s impossible to say but there’s definitely more than just Brexit at play.