In reply to MG:
To my mind, the answer is some combination of:
a) The regular commuters on this route will know about the issues at this junction and should probably be able to make some effort to avoid blocking the pedestrian crossing itself by leaving a gap.
b) If unfamiliar with the junction and/or inadvertently blocking the crossing anyway, watch the pedestrians and apply common sense. If they seem happy to walk around you, sit still and let them. If they won't/can't and you have space, move forwards or backwards as appropriate to get out of the way.
c) If stuck in the yellow box itself, and blocking crossing traffic, that traffic is probably going to allow you to get out of the way out of simple self-interest. Not sure what the highway code would officially suggest but obviously you're going to have to watch out for pedestrians and give way to them in preference to running them over. If you're near enough to the edge of the yellow box to not be blocking traffic, and didn't get chance to cross before other traffic or pedestrians started moving, probably prudent to wait for your next green light.