Wonder if the level busts could have anything to do with the floating transition altitude/level. I've always like the fix transition altitude/level being the same on the way up and down.
I would say that the low transition altitude and level in the UK contributes to some altitude busts since you reset QNH or QNE in the terminal area at times of high workload and traffic. Also, clearances to levels and altitudes near the transition layer are common, some companies reset the altimeter when a clearance across the layer is received, others wait until crossing the layer lest the controller ask for "passing altitude". Just when you reach for that altimeter knob, you inevitably get a vector or another altitude constraint, it's the nature of the game in busy airspace down low.
In the States with transitions at 18,000 you normally get a less urgent clearance involving an altimeter transition, e.g. out of 12,000 cleared to FL240 or descending out of FL200 cross BOBBO intersection at 11,000 and 250 knots.