The whole point of this signalling scheme is to prevent eye contact, or any engagement. To an observer who's unaware of the system, it just looks like a local embassy official out for a stroll. Or doing some other routine thing that a lot of other diplomats also do. Meanwhile, Gordievsky is probably never even at that bakery. Or he is, but at random times that rarely overlap with British diplomats in the area. The only reason he would visit it at 7:24am on a Tuesday would be to signal for exfiltration. If and he's doing that, then the risk is worth it.
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.