I know it sounds scary to put your passport in the post. I posted about this over the summer, and got some good advice.
1. Get a prepaid special delivery envelope from the post office (I think that's the name--they are silver, and cost approx. 5 quid)
2. Go to the nearest DVLA office and have them check over the documents and send them from their office. They will include the envelope with their package.
I did this back in August, and had my passport and provisional back in just over a week. Just to let you know, they come separately, so don't be concerned. I got my provisional back first, and then the passport the next day.