Your tag doesnt need to match the name exactly. You can use a J- Tag, or a [J] tag, or anything along those lines. As long as the tag isnt currently taken (check before you finalize) - I had that issue with the junker's union, which was originally labelled the Salvager's Union. I wanted to use an SU- Tag, but it was already taken by an ancient faction called the Slaver's Union which I had never even heard of.
As for the name of the faction, there's a few options here as well though I would advise against using the terms "Union" or "Congress" in the name - you dont want to be confused with the other junker groups. Just try to base it on something native to bretonia. You could be the Bretonian Salvager's Party, or London Labor Force, there's a ton of possibilities there.
Potential Terms:
Party
Company
Cabal
League
Syndicate
probably missing severa potentials but any of those would be suitable, depending on how you want to operate your group. If you're going to deal more in 'legal' business (trade, salvage ops, etc.) you could go with party or company. If you're going to deal more in criminal activities (piracy, smuggling, etc.) you could go with cabal or syndicate. League works for either focus.
EDIT:
Personally I think The Cabal would be a cool name =P