I'm going to tell a wrestling analogy now as its a very similar principle.
As I'm sure you all know, the HT of the Superbowl is a big deal in America where everybody tries to steal viewers. WWF were no different during an extremely popular time 15 years back called The Attitude Era.
Here they arranged a match at HT between two of their biggest stars, Mick Foley and The Rock. This was an "empty arena" match that was taped and shown "as live" whereby the two guys fight in an empty stadium and hijinks ensue as they spill out of the ring.
One of the participants, Mick Foley, wrote in his autobiography about his experiences in watching the match. He was in an airport bar during the broadcast of the Superbowl and asked the barman to switch it to the wrestling at HT, which he did. Initially there was some opposition to this but as the guys in the bar started watching it, they got more and more into the match. They laughed when Rock threw salsa into Foley's eyes and said "that was mild sauce, you baby". They were right on the edge of their seats when Foley kicked out of a Rock Bottom and started a comeback. As the match was ending people were actively enjoying it.
Then at the end of the match after Rock just won't stay down, Mick Foley has the brilliant idea of pinning Rock using a forklift and he gets the win.
Here's the problem; the crowd were totally into the match right until the forklift scene because they included a ridiculous camera angle that could not have been shot live. A full on close up of The Rock's face as the lift came down on him. Foley wrote that it killed the crowd in the airport lounge who were all complaining saying "how could they have filmed that?" and then started up with the "fake shit" that wrestling has to fight so hard against.
The point to this story is that this is a lesson that Top Gear needs so desperately to learn. Yes, we know that it's all fake but if you don't mention it then we're willing to suspend our disbelief to go along with the fun. However when you perform shots such as the Nova arriving on it's side after the supermarket run, you are insulting the intelligence of the audience and it jars them out of the entertainment into thinking "that's obviously fake". Both WWE and Top Gear rely on the suspension of disbelief and when you insult the audience, you break it and consequently their "buy-in" to the show.
This is why people generally think Top Gear is going downhill; it is becoming a show that is willing to break your suspension of disbelief in an effort to appear funny without realising that it CAN'T be funny if we look at it and instantly think "that's bullshit".
Here's that WWF match if anybody at all cares. That camera shot is at 18:30 or thereabouts.
[youtube]http://www.youtube.com/watch?v=qRBZuQK6jg4[/youtube]
Compare to the framing of Clarkson in this shot at 0:40. He is very obviously over to the far right of the screen and it looks off-putting, then when the motorhome falls it snaps you out of it
[youtube]http://www.youtube.com/watch?v=Yq6Qe4rFFJY[/youtube]