I have a feeling that you can't use the connections security in T5B/C unless your flight is departing from those buildings, I seem to remember reports of that before - and there is a significant risk that connections security in the satellites may not be open on the day you try this. I think I would stick with using connections in T5A tbh or head landside and back airside as suggested by cws - the former is less advisable if you are coming up close to conformance at 35 minutes.

If when you land you come in to B or C, and you see your gate for your short haul is B or C as well, then have a quick look whether the connections security in B or C is open as you head down towards the transit or walkways as it may be best to use

I am not sure I quite buy orbitmic's theory tho having walked "the wrong way" in the tunnels before myself. No one really cares if you look like you know where you are going.