Though it may constrain your options a bit, I would strongly suggest booking the London Eye in advance over the internet. The queues may not be too bad in September, but by booking in advance you can save a bit of money and also easily collect your ticket from a kiosk.

Alternatively, if you are traveling with someone, and have a rail ticket, you could take advantage of the 2-for-1 at both the Eye and the Tower (http://www.daysoutguide.co.uk/2for1-london).