How do I identify the common wire when both wires are red?
If by "common" you mean what North Americans (and I thought UK also) call neutral (blue, black deprecated, white North America)... A switch doesn't necessarily have neutral/common.  

A switch connects two wires: a supply wire that is "always hot", and a "switched hot" wire to the lamp which is hot when the switch is on (lamp is intended to be on).  

All the switch is doing is connecting those two wires, or not, depending on how you've thrown the switch.  Since it is only connecting, it doesn't care which is which.  

Since both wires are hot at least some of the time, it is correct to use a "hot" wire color like red for both.  

Where youm may have seen a neutral/common colored wire on a switch is this oddity. Much house wiring is done with twin-and-earth cable, and usually twin-and-earth comes in only one color pair (brown/blue, deprecated red/black, North America black/white). Neutral isn't used in a switch loop, but you're stuck with the wire colors available, so one ends up being black/blue/white even though it's not neutral.  However I have heard of twin-and-earth being made red/red for exactly this application.  Maybe this was required previously. 

In a similar vein they might have required (or chosen) metal conduit. Then they would use individual wires of appropriate colors, and the wires could be newer than the conduit. 

Up at the lamp, the lamp connects to switched-hot and neutral.  That is the only place neutral enters the picture.