
I have imported 2 girls in last 3 years.
If my memory serves me right most ferries we looked at do not allow foot passengers with pets so you will need a car to transport a pet.
To get to the continent&back we have traveled Harwich-hook of Holland, Dunkirk, euro tunnel and Harwich- Denmark.
My preference would always be the tunnel especially if traveling with a new dog, as you stay with them so know they are not panicking etc.
We did do Hook of Holland- Harwich with one of the girls when coming to UK and she stayed in the kennel which was Huge!
Kennels in a another ferry on the same route however were tiny and would have not even fitted a dog of my breed in them!
The Denmark route takes too long to my liking for a dog to stay in the car on their own when they have just left everything familiar behind. We used it on the leg without a dog when collecting one of the girls.
I would think 15 week old puppy will probably travel better than an older dog as they will sleep most of the way anyway :) as long as the breeder has got them used to traveling in the car.If they are used to being in a crate even better and should be ok traveling in any of the ferry routes really (except the Denmark one).
I have also flown a puppy out of UK and that seemed quite hassle free and quick option :) Probably cheapest as well when calculating the cost of ferries & petrol to the destination country. He was 11 weeks old & seemed totally unfaced by his trip when I took him out in the other end.
I was more comfortable flying a baby puppy than I would be flying a 10 month old as I think for a puppy, they will sleep most of the way and have not yet entered their "fear period"...
I feel a 10 month old could be traumatised by the flight so have driven to pick up my girls (+the cost of flying about 70kg of crate and dog is huge :)!).
Well at least in my breed they seem to go through a late fear period then and can act a little funny anyways (girls first seasons looming etc).