Ironically, any banner on the Slann still works and applies to the unit.
FAQ 1.8 Page 107 – The Battle Standard Bearer.
Add “If a Battle Standard Bearer is in a unit that Refuses a
Challenge and is subsequently moved to the rear of its unit, it
loses the Hold Your Ground rule until the end of the turn.
Note, however, that if the Battle Standard Bearer has a magic
standard its effects continue to apply as normal (it cannot be
‘switched on or off’)”.
Note that a decent TG unit should generally be able to hold up against a Brettoniam unit, except when charged. or when the Brett unit is a "deathstar" set up with a lord and paladin and grail knights. or questing knights The killing blow only works for the lance on the charge, so the second round of combat is usually a good time to take the challenge or take the challenge with a champ round one and then accept wth Slann round two and thereafter if you think you can survive. (A lord hits 2/3's but wounds Slann 50% and then 50% ward save, so the Slann can often take 4 attacks from the Lord at S4 and the one attack from the horse and average only 0.78 unsaved wounds per ropund, making it a safer strategy for up to two rounds pretty easily, or more if you can recover lost wounds). The key is to use skink cohort or skirmisher units to force either failed charges and to be unable to charge the TG or charge the skinks and overrun into mthe TG. Bretts, other than questing knights, are very weak (S3 and S4) when they are not charging unless they are a character that chooses a magic sword from the BRB or their army book that boosts S. Most of the Brett magic weapons are lances or first round of combat weapons (except Sword of Lady's Champ if Grail Knight, which are rarely taken; Birth-Sword; or Sword of Heroes which is situational and only works against T5 or greater models).
The TG unit remains stubborn if it declines the challenge and LD 8 without Banner of Disciple (89% pass rate) and LD 9 with banner of discipline (95% pass rate). That is pretty good with cold blooded LD. If you can survive the initial charge or battle, then a saurus unit or Krox unit in the flank will ultimately kill any expensive deathstar Brett lance unit.