I've used that junction, or passed it, hundreds of times over 20 years and I've probably spent 10 of those years commuting towards London and back, my thoughts are :
I can just about believe you could mistakenly turn onto the slip road by the 'T-junction,
but I can't believe you could drive 600 yards down the slip road at that time of day without 'meeting' traffic coming off the motorway and 'being alerted' that something was up,
I absolutely refuse to even consider that you could fight across 3 lanes of oncoming traffic (observe the traffic density on the screencam) without realising that you were doing something really wrong and extremely hazardous.
There's a wilfulness about this behaviour........
It is difficult to put that possibility completely out of mind.....
...the T-junction, fair enough
...the slip road, possible if all the oncoming traffic is in the left lane ready to turn towards STadhampton (does much traffic turn right at that exit?)
...the speed, maybe, as the slip road does have more of an appearance of a single carriageway A-road than a slip road as it does bend a lot and there is no indication that it joins the motorway direct until quite near the end
but then it gets harder to comprehend...although a point comes at which even if he did realise his error, his options were severely limited.
...is it possible that upon reaching the junction itself at speed, his access to the hard shoulder was blocked by traffic and the clearest route at that instant was to veer across to the left? Maybe, just!
...staying on the left is probably the choice rather than trying to veer back across to the right and the hard shoulder
...not stopping on the central verge, maybe if he feared losing control or the caravan rolling into traffic
..he seems to be trying to avoid oncoming vehicles, which suggests not deliberate and not some sort of medical condition,
but it is the speed that is the issue - maybe panic took over, maybe the accelerator was stuck, maybe, maybe, but if he had any control and any urge to avoid an accident, why is he still going at that speed even 100 yards after joining the carriageway????
It is just so sad, but so scary that this can happen, and while easy to dismiss as the actions of some elderly foriegner, it is still the case that some learnings need to be made on the back of this - clearer signage at that junction for one and also the Irish idea of "Wrong Way, Turn Back" signs flashing on the back of all signs on all sliproads....or even sensors on car satnavs that soudn a warning when cwertain no entry signs are passed?