I've not read all the post so forgive me if these points have been already raised.
Presumed liability AIUI is that the bigger vehicle is liable to be more responsible than the smaller one in the event of a collision,this isn't criminal law but civil.
Which means if an HGV collides with a car the HGV's insurance is automatically liable unless it can be proved otherwise,eg; if an HGV is driving down a road at legal speed and a car pulls out of a side road against a stop or giveway sign causing a collision,the HGV's insurance has a case,but not if roles are reversed which is pretty much the law as it stands.
But if there's any abiguity on on each side the civil responsibility onus is on the larger vehicle.
This state of affairs knocks on down the line, car v bike collision or bike v pedestrian.
The problem in the UK AFAICS is a total lack of traffic policing due to police focres across the country being totally under resourced and demoralised by a poor court system and lack of imaginative sentencing which has led to the police running around playing catch up or not.
This state of affairs has given the unscrupulous and downright criminal element of society free rein,hence people driving without car insurance,MOT or VED,not to mention the increase innthe number of drunk or drugged drivers and cars with false reg plates( so called ringed cars),and other motoring crime on our roads,as a result such drivers are less willing to stop in the case of a collision.
And also more likely to bully other road users who they see a invalid road users,but more often than not because such bullies are complete and utter gits who know the state of policing and couldn't give a monkey's for no one else.
A better equipped and better trained traffic police force and a more harsh penalty system whose implementers are willing to hand down maximum sentencing for repeat offenders would,IMO go a long way to solving UK motor crime,but when the tail(motoring lobby)wags the dog(government) and the same government over a period have reduced our policeforce by 20,000 officers and slashed it's funding to the bone,we end up where we are presently, at the mercy of morons both in government and on the roads.
One final point,anyone who thinks paying for motor insurance through raising fuel duty to pay for it will make drivers more responsible,is living on cloud cuckoo land AFAICS,as there's no direct comeback on any a driver who should be involved in a collision proven to be his/her fault,nor is there any extra insurance load for anyone with a string motoring convictions onntheir licence.It's just a bonkers idea full of holes IMHO