In reply to Tamati:
Cool. It's hard to offer advice at times when you don't know people's driving habits and intentions. My parents do the 700km from here to Calais in 3 days, whereas as I've been known to do well over a 1000km in just over half a day on a motorbike with no fairing
I was keen to get there
In which case, you can easily save yourself well over 100 quid by going for some of the more easterly routes (Cherbourg, Caen, Le Havre, Dieppe). Hard to give a proper comparison without more precise details, but let's take, Britanny Ferries, 2 people + car, 1 way, 10th July (Thursday). Portsmouth -> St Malo (only 1 crossing per day, overnight ~12h crossing, cabin not inc.) = £234. Same info, travelling Portsmouth -> Caen (quicker ~8h daytime crossing 3h+ more driving in France) early morning crossing £139, afternoon crossing £99. Condor's prices to St Malo seem comparable. These differences will be even greater if you compare with the more Easterly ports mentioned above, especially if you are travelling with a bigger vehicle (campervan/mobile home etc.). Please also bear in mind the day you travel, that same crossing above from Portsmouth to St-Malo, costs £199 on the Wednesday, £234 on Thursday, and £324 on Friday 11th. That's a fair bit of drinking money and galettes in my book. I've probably done these crossing hundreds of times now :-/ You can also save a bit if you're a member of the Caravan Club and book via them.
Toerag is right that there is plenty to do around here and Ploumanac'h has got lots of nice bouldering. Basically, over a week, you could drive about an hour a day, working your way around BZH coast bouldering or sport climbing in a different location/scenery every day. I've not done a lot of mountainbiking around here (which my very lonely Kona can attest to), but I know people who have, and there are plenty of nice bits of singletrack; so as long as you're not looking for some gnarly downhill action.
Around the Monts d'Arrée and Huelgoat would certainly be worth looking at. It's worth noting that Googlemaps thinks that lots of paths/tracks around here are roads, which can be both a good thing and a bad thing!
What you can never guarantee, is the weather; and the number of tourists varies greatly depending on the general outlook. However, it also changes very quickly, is extremely localised, and can mean a morning bike ride is a washout, but an afternoon climb on dry granite still possible (if the sun decides to come out and it is windy as it often is). Planning anything in advance is the tricky thing!
Hopefully some of the above will also be of use. Sorry I can't be of much help at all south of the Loire.