The form from HMRC is for essential car users who HAVE to use their vehicle as part of their job and either have a company car or a car allowance. If you are only doing the occasional bit of consulting the mileage charge is up to you. Worked with a guy 20 years ago who charged 60p mile for a motorbike for private investigation work (we only paid him 20p for the work he did for us).
Part of the excuse given for the mileage charge staying steady for so long is the engines are more efficient now. Doesn't help much if you are running an older less fuel efficient model.
Do make sure your insurance covers this type of use.