The arms and armour listed for various armed followers seems to be tuned for the Uther/Anarchy phases. Is it the intention that these followers should come with better 'default' kit as the campaign progresses in to the later phases or are PKs expected to have to pay if they want their people to keep up with the escalation in equipment?

Regards
Luke

The arms and armour listed for various armed followers seems to be tuned for the Uther/Anarchy phases.

Yes, at least for knights and cavalrymen


Is it the intention that these followers should come with better 'default' kit as the campaign progresses in to the later phases or are PKs expected to have to pay if they want their people to keep up with the escalation in equipment?

In general, yes
The protection for most of the infantry doesn't get much better as time passes
Cavalrymen tend to be 1 step behind knights
Knights will attain the armor described in GPC during that Period