If the speed is too high for a Speed Awareness Course or COFP,the 2 factors that will affect the level of fine are how far over the limit you were, and your weekly income.
The fine will be a percentage of your weekly income, with a discount for an early guilty plea.
In effect, it won't make any difference to the fine whether you attend in person or allow the Single Justice Procedure to run its course.
If the speed is so high that the single Justice is considering a ban, you will be notified to attend in person.