Can’t really recommend a cottage but we quite often stay in the Merrymoor at Mawgan Porth. Dog friendly and a huge dog friendly beach right on its doorstep. I’d imagine quite a lot of cottages near to it would be dog friendly as it’s very popular with dog walkers.
One of the nicest I’ve stayed at is in Norfolk - in Happisburgh (pro. “Haze-bruh”), and is called “The Crib”: very dog-friendly, large, secure garden with 6’ fences for the dogs to roam, and 3 minutes walk from the excellent “Hill House Inn” - lots of clifftop and beach walks, as well as some inland routes; great fishmonger in the village too, although not much else aside from a small and rather sporadically-open general store.
Winterton is also a good call - lovely spot and fab beaches for the dogs.
The Northumbrian coast - from Alnwick up to Eyemouth is also beautiful and generally very dog friendly. Can’t recommend a particular place as so far the ones we’ve rented have not had good gardens for wayward dogs.
The gardens are something you really do need to research carefully, because most people’s idea of secure really, really fucking isn’t if you have the sort of dogs we have!
Also, for godsake avoid the rental company “Sykes Cottages”, because they are fucking shit.
In Alnwick high street a lovely pet shop would leave biscuits and water outside for passing dogs, Barnaby demolished the lot at break neck speed. We went into the shop to apologise and offered to replenish the bowl of biscuits, to be met with ’ it’s a Flat Coat, only to be expected ’ and a chuckle. Needless to say we damn near bought the shop up. Such friendly folk.