Not everyone in the great hall would bow beneath Lelouch's tyranny. Charles's lingering influence was still formidable. Lelouch was merely some kid who had appeared out of nowhere. Even with the support of Schneizel, Bismark, Xiao Ran, and the others, these opponents still had the confidence and courage to step forward.
As a nation with a noble system, governed by nobles and with its military controlled by nobles, Britannia had many aristocrats with immense power. Their connections and interests were deeply entangled, and their influence and infiltration throughout the country were extensive. This was why, in the original storyline, Lelouch had decided to abolish the noble system. Although the nobles were the core strength of the nation and had contributed much to it, they also sucked away its nourishment like maggots. Britannia was currently at war, and all its conflicts and interests could be directed outward. But once Britannia could no longer advance, these nobles would become its greatest cancer. For profit, they could do things that would leave people dumbfounded. Not every noble truly regarded the country as their own home.
Some of those who spoke up and stood tall now might genuinely have remained loyal to Charles. But others were acting for their own interests, relying on their families' influence and control over the country, believing Lelouch would not dare lay a hand on them.
Lelouch was a stranger to them, but from what he had just said, they could sense that, in some respects, he would be even more domineering than Charles. Charles's control over the country had indeed been strong, but he cared more about foreign wars, invasion, colonization, and control over the nation as a whole. That had left many gaps for these nobles to exploit for profit. Those profits were things Charles did not