The Stonehenge segment is outstanding, but the setup for the visual comedy is so complicated that you would burn up your full time allotment if you used it. A good fast quote that you should use is right after Tony Hendra, as their manager, explains to them that the rival musician can use a cover "that is even worse than the cover we wanted for 'Smell the Glove'" is because "...he's the victim." The quote is "Oh, I see. There's such a fine line between clever and stupid." Another good quote is from Mick Fleetwood, their then current drummer, when he is in the bathtub wearing a shower cap and he says, "Well, with enough sex and drugs and I could live without rock and roll."
I'm not sure what point the clip with Bruno Kirby as the limo driver would lend, but I don't know how much of that scene you intend to use.
I assume you are going to select more material than you can use, then cut it altogether several ways and lose the bits that don't fit in well with the rest. Pity you don't have a few more minutes, because "This is Spinal Tap" is rich in segments that could be excerpted, but I assume one of the points of the exercise is to teach you what will stand alone and what wont.
Good luck, and I suspect you will have some fun with the exercise.