I must admit I've never really thought about it before however, when the 455's were built there was no other train in the southern area that the 455's could couple to using the jumpers as 455's are built with tightlock couplers and the only other stock at the time to have a tightlock coupler were 508's and they were already set to go up north once the 455's were out in service, not only that but 508's don't have control jumpers on the front they have a electrical box. All the other stock at the time was running around with the drop head buckeye's and had different brake systems so the pipes were different.
My understanding (and this is just my thoughts) was that the 455's were supposed to be the first of a new design of multiple unit for the southern reigon and all 3rd rail trains were to be built on the same sort of spec/design but the only 3rd rail units that BR built after the 455's were the 456's which did also have a tightlock coupler and control jumpers and were able to work with 455's. After the railway was made private and everything split up, the building of stock was given to private companies which decided to stick with auto couplers and electronic boxes, 458's have a tightlock coupler but have a control box like the 508's, this design of coupler is better as there is no safety issuse of someone having to go between the units to put up pipes and cables or to remove them to uncouple a pair of units. During the Network South East years they drifted away from tightlock couplers and cables as they decided to create the networker family so the 456's were the last jumper units on the southern.
