Top Gear is very twitter friendly. Normally their tweets consist of a quick sentence that has some kind of wit or hook to draw the reader in, then a link to their website. Actually, all of their tweets use this format. The only difference is that 19 of the 50 tweets I reviewed (38%) used a hashtag of some kind.
To make their tweets more effective, I think that Top Gear needs to give them more variety. While each tweet might be witty, following them for a little over three months now has become annoying, and posting a link in each tweet makes it look like spam. I would make tweets that were witty but not necessarily related to anything on the website. Maybe tweeting something like traffic reports on a local highway could be seen as funny.