I looked up acronyms for PU, the only one I could find relating to smell would be pig urine. But all of the other info about it I found said that it doesn't actually stand for specific words starting with the letters P and U. Here is the best answer I found from the website- The Phrase Finder:
National Transport Authority > Taxi and Bus Licensing > Taxi > SPSV Driver Licensing > Renew an SPSV Driver Licence. A regular SPSV driver licence is valid for five years and is renewable; a local area hackney driver licence is valid for three years and cannot be renewed. (The holder of a local area hackney driver licence who wishes to continue operating can, however, apply for a new licence.) To renew your SPSV driver licence, you must hold: 1  A valid full Irish driving licence or equivalent; 2  A valid SPSV driver licence; 3  Must be Tax Cleared with Revenue.
Your Provisional licence does not expire until your 70th Birthday. However, you have to renew your photo part of your licence every 10 years, so your photo can be updated. There are a lot of rumours about the driving age in the UK increasing to 18.