The coupling issue is one of electrical difference. They will work quite happily together (and have done so around the yards) but the problem arises when someone presses the Call for Aid in a 456 and it's being driven from a 455. The PIS/CtC/CFA module fitted to the 456s is a very different spec to the SWT 455s, and the Call for Aid is integral to that new system. The small mods done at Wolverton will undo this change, and the Driver of a 455 will quite happily be able to detect that someone is pressing the big red button in a 456 on the back (or vice versa).
Not a "can't do", a "won't do" until it's safe.
Not a "can't do", a "won't do" until it's safe.
