Dogs are OK for scaring off casual criminals but if someone really wants something they are not much of a problem. Bit of doctored meat and they'll sleep like a baby.Definitely big dogs, though I am not sure the big mastiff breeds are the right ones.
There was a time when Britain was famous for the pedigree working dogs it produced but no more. I have three German shepherds off working lines, a mixture of Czech, DDR, and Slovak. Max is getting old and it looks like Eastern Europe is the best place to get a replacement, there used to be a good Facebook page but that has been closed down.
Don't forget, a dog still needs to be trained. Mine are safe if they are out when the postman or delivery arrives, but it would be different if I said the word! They really really don't like dogs, but I think several are needed as it's too easy to disable just one. And a radio controlled catch on the kennel gate would be handy.