They couldn't because the action was in two parts, the first that the tree was part of the highway. They had to defend that as their liability would have been automatic under a highways act. The Judge ruled that the tree was not part of the highway. He seems to be a smart cookie in assimilating all the points even though they were not familiar ground.
The second part of the action was proven but it looks like it was because of omissions of recording rather than the other factors, just as in the Cavanagh case he ruled this was a high risk tree that should be inspected annually. I do not think the Bing images are available but there are streetviews which suggest the tree, presumably planted when the A45 was dualled (why plant a tall forest tree in the central reservation) so not veteran like the Cavanagh case, had dieback for several years. Being lime the dead branches would have been readily shed annually removing symptoms and probably not appearing worse other than misshapen.. Even a closer view from the central reservation may not have noted problems behind the plethora of sucker shoots but K Deustra is difficult to notice and limes have a poor ability to isolate compartment one breaches as they only exude a waxy bladder into the vessel rather than block it with tyloses.
It looks like the HE had delegated all their responsibilities to Amey, who in turn had sat, Carillion like, handing out work and taking a cut with neither client nor contractor keeping records when public money was spent,
Had a proper tree inspection taken place [1] and been recorded with the inspector being able to defend whatever decision had led to the tree still being there then HE may have been able to ask for some of the responsibility to be apportioned with the driver. After all we don't get to see, as no post accident witnesses were sought, whether the tree was struck or did it strike the car; why was the car in the outside lane, it was not a tall tree and the damage would have been less 10ft over. How fast was the car travelling and what were the conditions. Mind this didn't cut ice in the poll case as the drive by inspection wasn't accepted as being adequate nor by a suitably qualified person.
The judge also severely criticised an arb witness in both Cavanagh and this case because they were not objective and/or economical with the truth. So in both cases their testimonies were ignored.
[1] I did undertake rudimentary inspections prior to 2013 but they were simple above ground non intervention eyeballing, always within a reasonable distance and most often all around the tree. I had 2 minutes per tree on average over 5k trees in 170 locations. but my notes were typed up and photographs taken and synchronised with a GPX trace. After the Cavanagh case I declined to do the work again as I have no arb related qualifications, mine being mostly competencies to do the work but I did attend the basic Merrist Wood tree inspection course and my professional liability insurance was in place and my insurer accepted the risk knowing I was undertaking the work.
The inference from these three cases, Poll, Cavanagh and this one is that HE has not taken any of the lessons on board plus tree inspections are going to get much more expensive and we'll all be paying for that in taxes.