why is car 2.5tdi tax on emissions higher than a (lorrys 7.5 ton). tax.?

Long shot possibility that the lorry has lower emissions. More likely that they are dinging commercial vehicles through registration and fuel taxes. Also, commercial vehicles are likely to be subjected to emissions and safety inspections on a much more frequent basis that cars.

As a former Juggernaut driver, I risked being inspected several times per day while for my personal vehicle, safety and emissions inspections are only every 5 years.