It reminds me a joke about Soviet spy arriving in Berlin and caughting stares from everyone around. What's wrong with my cover? Is something gives me out? Maybe it is a parachute? Or AK-47? Ah, it should be blue Slavic eyes! And he wears a sunglasses just in case.
I took it to mean that the MI6 agents were under diplomatic cover. It would not be strange for British diplomats to carry or eat British things, right? And it's also a decent defense against an accidental signal, since non-agents would naturally not have Mars bars nor bags from Harrods.
It's also only once a week that the timing is enforced, so they can vary things up throughout the week to hide that pattern in noise.
And the candy bar/Harrod's bag doesn't come into play unless Gordievsky needs an exfiltration. So it's not like they're noticing this weird thing where British diplomats always eat Mars bars every Tuesday. It would only happen once at most.
They know they're being surveilled. They can't change that. So they make their signal as mundane as possible.