HMRC states that the initially agreed upon salary sacrifice value needs to be committed to for a minimum of 12 months. If you chose to opt out beforehand, you may be required to return the tax savings (except in the exceptional circumstances, such as being made redundant or you or your partner falling pregnant).
Can I increase or decrease my salary sacrifice total?
You can increase the amount within the first 12 months but you cannot decrease it until after the first 12 months.
For the reason stated above, you are able to to add extras to your salary sacrifice package, such as increased mileage or additional drivers.