Haven't read the Magna Carta recently, but I'm going to take a leap and say that it does not contain a definition of "driver" for the purposes of s. 172 RTA 1988, s. 89 RTRA 1984, or at all.
Many of the whackos like to rely on Black's legal dictionary. By selecting the correct out of date version they can find "driver" defined as someone "employed" in driving. Apart from the fact that a dictionary has no authority, and it's a US dictionary anyway, the laughable thing is they don't notice the definition of "employed" in the same book.