I have 2 GSP's and I'm going to have to say it's definitely not part of the mix. Even though he has the ticking on the legs, you can still get that from the Brittney (Which I have as well). +JR / Brittany mix.
you've gotta be joking... a brittany spaniel? you think the dog in the op is
part this?
this +
this =
this
there are only a few dogs that have a tail with a white tip... it's trademark for jack russells and basset hounds. it's so that the hunter can see where the dog is in the brush... the white tip on their tail acts kinda like a flag. they also carry their tails high, not low, like most dogs.
the dog in the op obviously has a tail characteristic of these kinds of dogs, but the only dog (that i can think of) that small with that size and that tail is a jack russell terrier.
the op dog totally has trademark border collie markings, so that's where that comes from.
therefore, border collie/jack russell mix.