Substitutiary Locomotion is the same spell as Piertotum LocomotorThey both do the same thing - bring empty suits of armor to life to be used in battle. Which leads us to...
Miss Price is actually Professor McGonagall
Substiutiary Locomotion doesn't just animate inanimate objects with energy.All the items we see under the power of the spell seem to have odd personalities and quirks to them, and for the ones whose owners we know, these seem to relate either to their owners' personalities and quirks or subconscious feelings and desires of those owners. Professor Browne, who is a cheeky fellow but also likes Miss Price a great deal, has a pair of shoes that boot her in the rear; meanwhile she has feelings for him she is denying/suppressing, and her nightgown approaches him to dance (and chases away the vicar whom she is not interested in). Carrie and Paul, who just want to have fun, are seen dancing and playing with their own clothes—except when Paul is being 'disciplined' by Miss Price's pantyhose (she had just warned him to calm down and stop making a ruckus). Charlie, meanwhile, gets to be on the receiving end of his own troublesome, rebellious nature courtesy of his Sunday pants. Even the vicar, who harbors a not-so-secret desire to voyeuristically pry into Miss Price's life so he can romance her, has his hat go flying inside to investigate. This implies, then, that the suits of Animated Armor are not merely animated by magic, but by the spirits of the dead soldiers who once owned and wore them; this, as much as the age of the armor, would explain why they responded to the ancient, old-fashioned way of speaking the spell words instead of needing the 'modernized' song-and-dance number, because it was familiar to them. This also explains why the armor gradually sinks to the ground when the spell is broken instead of simply dropping, to visually signify the spirits returning to the Other Side as their armor "died" again.