The p stands for provisional. They have legal significance in Northern Ireland, where drivers are still subject to a speed limit for a period after passing their test, but not in the rest of the UK. There are about 30 million drivers in the UK, so there are probably at least 30 million opinions on whether they are a good idea and how long they are needed. Personally I think they are not needed, because just after passing your test, you are probably among the safest drivers.