If they were overtaking when the tyre blow out happened, which the police seem to have said, then the only person who can say they were speeding is the person being overtaken.
Surely they have been asked about it, if not why not and did they even stop. All we have heard about is the two lorry drivers saying they passed them 5 mins and a few moments before the crash. Both say they were driving calmly.
But the speed is probably going to be above 70 mph on that road and overtaking, just like 99% of people who overtake in the UK on the motorway.
Crash site, skid marks with a blown tyre and a lambo made of jelly is always going to look worse, especially if they braked straight the way instead of trying to slow down gradually