A few considerations:
- Are the flight times all more or less satisfactory for you?
- Which airport is most convenient? Royston is closest to STN and LTN, though a train to LGW (connecting at King's Cross/St. Pancras) is certainly possible.
- This being FT, have you considered any other options, particularly those that might earn miles (eg, A3)? Together with a more convenient airport, this may be a deciding factor.
If the EasyJet schedule hasn't been released yet, it is unlikely that prices will go up substantially. Other airlines may also adjust their S17 schedules over the next few weeks, giving you more options and/or price competition.