Since getting trade plates (for the first time) involves an interview process, I doubt that it will be quick but your local DVLA office should be able to advise you:
http://www.direct.gov...gContacts/DG_10012974
(Your local office will also know far more about taxing vehicles than any Post Office will).
In the meantime, read the guidance notes
http://www.dft.gov.uk...guidance%20notes.ashx
and fill in the application form
http://www.dft.gov.uk...20licence%20form.ashx
Chris



