Strict is an understatement -- apparently some of the gauges at LHR are sized wrong and are slightly less than 45x35x16 cm, causing bags that are exactly that size to be rejected. I saw this happen to a BA Gold in the F lounge, who had specifically bought a new bag at a store in LHR to fit the new size, and it was rejected by BAA ... needless to say he was livid with BA (the wrong party) about this problem (and of course BA was soothing him by saying "nothing we can do about this sir", which was doing wonders for his complexion).