The Highway Code's quite clear on level crossings; "Never drive onto a crossing until the road is clear on the other side and do not get too close to the car in front. Never stop or park on, or near, a crossing.", so in theory a yellow box is quite superfluous, as possibly counterproductive as box junctions do have an exception, "However, you may enter the box and wait when you want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right.", maybe a 'yellow box' level crossing very close to a junction could raise issues there, and as is said, given yellow boxes get little respect at road junctions anyway I doubt it'd make much difference, if any.