No reason guards shouldn't be patrolling 455s. There are door controls in all but 1 carriage on the 455/7s. There are some stations where dispatch has to be from a certain location, but that's the minority of stations. 456s are more difficult due to the lack of gangway.
I would add, for clarity, that Southern 455s (worked by conductors primarily on the Horsham and Guildford routes, but also elsewhere at times) do not have gangways between units (as previously posted) or any conductor panels at all in the saloons. In addition, dispatch is required to take place from the front cab of the rear unit
only when in an 8 coach formation, except in the highly unlikely scenario of neither the communications system nor the door controls working in that cab, in which case the rearmost cab would generally be used, or the rear unit taken OOU with the rear cab of the front unit used.
It is possible for a conductor to patrol a Southern 455, but realistically only when you have more than about 3 or so minutes between stops. Even then, if you come across something more complex such as a confused passenger or ticketing irregularity (bearing in mind Southern conductors are all revenue trained), that will be barely enough time to exit the cab, find and sort out the issue, and get back again to release the doors and dispatch. You can take this as factual from somebody who has 455s on their licence...
