It is NOT 'Give Way', it is a give priority as per the Traffic Signs Regs.
In the absence of an associated plate defining the area, the priority area is defined by reference to the infrastructure realities i.e. the bridge.
This is supported by reference to the sign (you have priority over) from the other direction.
IMO, I do not see how the contravention could be proved by reference to a vehicle not in the priority area. If this is permitted, then the Signs Regs are kicked into touch and, to continue the metaphor, who is to decide where the goalposts are placed?