Such an easy answer..
ever noticed how sometimes the stig is tall, sometimes short, somethimes alittle chubbier, and sometimes skinny as hell?
that says everything right there. how can 1 person drive such different super cars effectively enough to represent a whole companies product on public television.. simple, the companies provide the drivers...
for example. drive a lambo, then a lotus, these are two completely different cars that can take months to completely master. so in all fairness, in order to get the best lap out of each car, the drivers must be used to these specific cars.. Remember when the stig drove the F1 car and got an awesome time? how the fck? even when they proved hwo hard it was to drive one of those.. it would take months to get used to driving a car like that, and theres no way top gear spent months on a single episode especially using something as expensive as an F1 car.. remember american stig? ethiopian stig? all jokes right? but essentially hints at who the stig really is..
Lets say topgear is reviewing a ferrari. Ferrari provides their own driver (and car) and they wear the stig suit. same goes for each and every car company out there.
in short.. the stig is a suit, and the person behind the suit is essentially a driver for whichever company the car is from.
P.S.... hiring an f1 driver would cost millions of dollars.... theres no way BBC could afford to get an F1 pilot..