Druidic Lore
A note in response to further queries: the classes listed above are not intended to be a hard and fast limitation on classes available to players. They simply show the classes that occur "natively" in the campaign setting.
As mentioned, warlords are most likely to possess classes from this list (excluding druid and sorcerer), as any warlord needs the support of his warriors (ie, the common people), and the superstitious commoners are unlikely to follow a warlord into battle who displays strange mystical abilities. Warlords rule by sword-right, taking and holding their lands through strength of arms, and few warriors would willingly be led by a leader who disposed of his foes with magic or stealth. Above and beyond the prevalent fear and hatred for any arts viewed as arcane, such methods are seen to be cowardly, and no true warrior follows a coward into battle.
Druids are also highly unlikely to become warlords. The druids are a class apart, a social caste unto themselves, owing allegiance to none. While druids serve warlords and warchiefs as advisers, magistrates, scribes, and other roles, they ultimately swear fealty to neither clan nor tribe. Druids are the only class of society that are exempt from military service as a civil duty, though they may choose to serve if they wish. As they are a hierarchical organisation of their own, they will very seldom go against a druid in a rival warlord's hall. Far more likely both will remain aloof from direct conflict. Even the nobility treat druids with the respect their position accords, and the spilling of a druid's blood is generally held to be taboo. Even the most powerful king would hesitate to risk incurring the wrath of the entire order of druids, for he would at best lose their service forever, and at worst find himself dragged away in the middle of the night to Inis Mon to be "experimented" on...
That said, exceptions can always be made. Clerics, wizards, and paladins do exist in the setting, but practically all of them would come from foreign lands where such things are more common (eg, Rome, of course). This would, obviously, seriously hamper their ability to rally enough support to ever become a warlord among the Britons!
So mostly the limits exist to give you a sense of the setting, and to force out any truly fantastic class choices (say, samurai or the astral deva racial class from Savage Species), but anything not too outlandish will probably be permitted.