The two sights are supposed to be quite independent. The bren Mk1 had the dial sight, not the fold down flat (what we call ladder type). This particular gun is a 'dovetail rail' gun which was meant to accept the normal British sight (the one at the back) the Czech type (the one forward) or a telescopic sight.
They were next to useless and were expensive to machine so weren't fitted on later guns, only the rear dial sight remained and then when the Mk2 came along, the dial sight was abondonned for the ladder type.