The pens come in packs of 3, at 4.99 each, and they're BOGOF. That's 6 pens for £4.99. You can choose sets of black, blue, or multi-colour pack (one black, blue, red). You can also get student discount, so if you're a student, then it's 4.49.
There's also BOGOF on Uniball Signo pens, but they cost £6.99 for a pack of 3.