As per Government rule, this model will be classified as a van as it weighs more than 2040kg

They are subject to the same rules and speed limits as cars on the motorway, and would only have a speed limiter if the owner chose to have one fitted (my employer have 70mph limiters on all their vans and pickups) A car someone else estimates as doing about 75 to 80mph wouldn't be judged as speeding by police as without a speed gun or calibrated police Speedo it's within the margin of error of being at 70.

I don't understand from the description how the accident happened and what relevance the exit off lane 1 has - unless it was a very minor collision, the pickup isn't going to be able to continue right across after the accident and turn off the motorway, so there's no way of knowing if that was the Ranger drivers intention. Was the pickup cleanly overtaking the OP in light traffic and pull in much too soon (fairly unlikely for someone to do in my opinion), or were the cars in lanes 3 & 4 going much the same speed with the OP in the pickup's blindspot (which is quite large on a Ford Ranger!) for some time before the accident? This sounds more likely to me, and will be why the insurance says 50:50, as there's a possibility the OP will have driven into the blind spot from behind (i.e. by partially undertaking the pickup) in the lead-up to the accident as the speeds of both lanes varied.

My car has advanced feature where it will not allow me to drive into another lane when there is someone in blindspot. Even If I do it without any indicator, My car will drive me back into original track. I have to override any of those warnings if I need to move from Lane3 to Lane4.

That's not an advanced feature it's a death trap, never heard anything like it.

I know these lane departure thingies make all sorts of fuss if you stray over a white line without indicating, they're a PITA, but had no idea the blind spot equipped versions physically prevent the vehicles from going where the driver points it.