I spent the last few years of my working life as a truck driver, a considerable part of that involving the haulage of sea containers.
In the UK the authorities come down so heavily on overloaded trucks (and their drivers) that, if you are a semi-intelligent trucker, you get paranoid about your weight!
In the UK (and, indeed, most of the European Union), the maximum permissible gross (ie all-up) weight for a six-axle truck is 44 tonnes (ie 44,000 kg/97,003 lbs).
This (just-about) enables a fully-laden, 40-foot container to be legally carried. The maximum gross weight of a 40-foot box is usually 30,480 kg (67,200 lbs to you old-fashioned Yanks).
So, normally, you can haul a fully-loaded 40-footer and stay legal in Europe.
If you are even a moderately-experienced truck driver you can tell by the feel of it if your truck if it is grossly overloaded.
So where are these instances of boxes bending/bottoms falling out under the weight of their loads happening? I would venture to suggest that few, if any, of them are emanating from UK export loads taken to the ports by road.