Since Mark is already seated in the front of taxi P, we have 7 friends left to arrange.

Jon and Sarah are sitting in the back of taxi P, so we treat them as a single unit initially. This leaves us with 5 units to arrange: Jon-Sarah, 1 more person in the back of P, and 4 people in taxi Q.

First, choose 1 more person to sit in the back of taxi P from the remaining 5 friends: \\(\binom{5}{1} = 5 \\\).

Now, arrange Jon and Sarah in 2 ways (Jon-Sarah or Sarah-Jon): \\(2! = 2 \\\).

Arrange the 4 people in taxi Q: \\(4! = 24 \\\).

The total number of arrangements is: \\(5 \times 2 \times 24 = 240 \\\).

Since there are 2 possible ways to arrange Jon and Sarah, multiply by 2: \\(240 \times 2 = 480 \\\).