You can only do it at a dvla office if there is no current owner ie its been registered into the trade. If there is an owner of the vehicle on there file, it takes about 4 weeks to get a log book as they have to write to the existing owner to see if they have sold the vehicle. If there is an owner you will not be able to get a tax disc for it. I've been selling used cars for 10 years and havent found a loop hole yet to get round this so if you find one please let me know lol