The reason is very simple. I remember a programme in which Jeremy said that BMW was expecting them to test the new 1-series, but that they decided not to test it. Everybody laughed of course, but without knowing the real reason of it

.
The scaglietti was tested. Not only in Top Gear but also in Fifth Gear, at the same time as BMW 645 ci. I think it was Richard Hammond the driver.
And the Land Rover Discovery...are you joking?

. Jeremy has tested and has compared it to the Range Rover (his favourite 4x4).