Some of our crews came up with the solution to this problem.
Its quite easy really. As 'cargo boy' says, you may not be insured with your own car and must always check that you are covered on the insurance provided with the hire car.
Yes, you are quite within your rights to refuse to drive if fatigued as per the Health and Safety laws. You may also collect your P45 the next day!
However although your company may stipulate the average journey time, there is nothing in the Ops Manual to say how many bathroom breaks and en-route rest stops you may take and for how long. Also Britains road's are becoming horribly congested, aren't they?
So your 1.5 hour journey suddenly turns into three hours through no fault of your own. Perhaps even meaning you are unable to have min rest before your next rostered flight!!
After this happens a few times the company might start putting taxi's back on.
BTW, pigs CAN fly! Have you seen those helicopters they have these days
(OK, sorry!)