Sorry, not £15 for simply a pair of wheels
Its £15 for two good-size, high-traction, great quality (as used by Razer and Behemoth) wheels that come complete with nut inserts to allow you to screw them straight onto cordless drill motor shafts.

Go to Homebase and pick up some large (178mm) wheels with average traction, moderate quality and with nothing more than a simple hole drilled through the middle and you'll have spent more than £15.

That said, it is perfectly possible to make very cheap wheels out of wood and bike tyre, or to pick up second-hand wheels from somewhere that you can adapt to suit your bot.