As the others have mentioned, many airlines use a "sliding scale" with prices increasing at the flight fills. Therefore it's an advantage to book early. However, the cheaper tickets are usually non-refundable and non-changeable so choose carefully. Be sure to read the Terms & Conditions on the airline website, especially related to luggage as budget airlines can be very strict about that.
I normally use easyJet whenever possible and will be flying with them again this year. I prefer using LGW when possible as transportation from London Victoria is easy. If you decide to use a flight from LGW, you'll need to take the small people mover to the terminal used by easyJet (only takes a few minutes).