
Generally you should contact your bitch's breeder, as if they are reputable they would be most upset if you rehomed her without reference to them. Breeders tend to network and are the best people to know of a suitable home, or be best placed to vet one.
If the breeder is of no help then contact the mastiff cluv=b, as I am surte they will have a welfare and rehoming section, again with experienced people best placed to home such a dog correctly.
You can get the clubs details from the Kennel Club Tel 0870 606 6750