I don't know of any animal that will eat bracken, rushes or brambles, your pigs however will turn over the ground and knock them back a bit. The rushes will come back once you take the pigs away, spreading lime before anything grows may see them off. As to the brambles, keep taking away the stems and roots as the pigs dig them up, it will stop them regrowing. Bracken is a hard thing to get rid of, it has a big network of roots underground, the pigs will dig some up and if you keep pulling it out it it may eventually give up.