As the 456s are 2 car and the 455s are 4 car, it wouldn't be a bad idea to send 455s and 456s North in lieu of 319s as that would then allow 2, 4, 6 or 8 car formations opposed to 4 car or 8, with 8 not being possible due to infrastructure restraints.
Interesting Freudian(?) slip there? The two classes you mention are both third rail only and the 455s are (much!) older than the 319s....
I know you meant 465/466 by the way... While I know that 465s were successfully converted to 365s I am unsure if anyone has even tried to do the same to a 466....