The Darlington Nurses set up a union, I don't know if it's open to all 'trades', I hope so because they would get a much bigger membership if they allowed anyone in any industry/profession to join.
There's also the FSU, if you were to get into trouble for expressing an opinion they could help you with advice, and if it's a free speech issue they can act for you, but you do have to pay legal fees with them.