Cancellation charges do apply if you cancel more than 7 days after the pre paid booking was made.

Mods will either cost more or less depending on the modification. if more then they bill the difference, if less they refund the difference.

IMO - don't book too early for pre paid rates especially, if you think there may be a possibility of the trip being cancelled.

You also need to bear in mind the dollar / pound rate at time of booking.
is the rate good or bad etc at time of booking -

Booking too early could mean you miss out on Hertz sales etc.