It has been interesting reading this debate, it reminds me of many I have had in the past, both within the UK, within Europe and within the USA. Clearly, as other have said, the pilots involved should have paid much closer attention to their ATC clearance. If there is one thing you DON'T do, it is to fly below your MSA.
I have long advocated a higher TA in Europe (18,000ft to harmonise with the USA and above Mt Blanc would be a good compromise) not only because of the harmonisation issue but, even more important, to raise it to an altitude where things are less busy. Others on this thread have mentioned the complicated SIDs out of Heathrow - turn here, turn there, level out until past xxx, then climb to yyy, change frequency to zzz, etc, etc. I have always thought to change altimeter setting in the middle of all this is only to invite yet another mistake.
And this brings me round to the subject of standardised terminology. This is not a case of the USA versus Europe or anywhere else in the world for that matter. When I used to fly many years ago, in different parts of the world, different phrases were used to clear aircraft to line up and then take-off (there are others too). And incidents were caused by such things. If we are really interested in safety we should work towards removing as many as possible of these differences.
It is not a case of just saying, 'pilots should take more care' or from a state ANSP's point of view, 'this system is best for our country'. However, professional a person may be, they are human and, as we all now, humans make mistakes.
So, please let us remove yet one more possibility for error and thus make the total system safer.