How far in advance to book ferry and campsite

Brittany Ferries sailings for the following year normally go on sale June / July. Last year, they were released earlier, May, I think. 
If you want a dog friendly / wheelchair accessible or a “ fancy” cabin, I would book as soon as the crossings are released, they get snapped up quickly.
Otherwise, doubt if there’s any real rush.

Enjoy planning 🙂

From 14th July (Bastille Day) to the end of August the French go on holiday.  It is the most busy time and booking sites is almost essential.  Out side of those dates there really is no need to book sites. 

BF prices also reflect this busy time.  In normal times the best time to book BF is one month in advance of your chosen sailing day.  I'm told that is when the dynamic price levels are at or around their lowest.

Like Pliers  I would advise  you keep an eye open for the timetable release and if you want to be sure of a particular cabin type book ASAP

Stu, Look now (May 2022) at Brittany Ferries to see what availability they have for July this year - so two months in advance. That will give you a guide as to whether you can make a similar ferry booking two months in advance next year.   B/F operates five routes to France - look at them all and you will see which ones are already fully booked and which ones are not. I suspect that if there is availability  two months in advance this year then there also will be availability next year, especially mid week, and especially in the daytime when you don’t need a cabin, rather than overnight when you do.

As for sites it depends absolutely and entirely on where you want to go. The dearest and most elaborate sites fill up fastest. If there is one site that you absolutely must go to then book it, but if your French is good and if you are confident to phone ahead a day or two in advance to smaller sites away from the  honeypots then you can tour without booking - even in July and August - provided you are flexible. We did it for half a lifetime.

You will gather I am not keen on advance booking ! 

I thought the principle of dynamic pricing was that as you got nearer to sailing date and the boat was filling the price went up.

The latest news on the new TT is June. I would imagine July 23 will be busy and so I would book fairly well in advance, like this June or July. But I like certainty.

Note there are 3 classes of tickets, distinguished by how much you have to pay up front, how flexible they are for making changes to your booking and the charge for doing so and how much you get back if you have to cancel.