p牌超速一問..收到兩封信控告我冇掛p同超速

If your speeding is more than 15km/h, plus no P plate, your p licence will need to be cancelled. If speeding is less than 15km/h, your p licence need to be extended. So, don't try to get full licence, otherwise waste of money. You may go to http://www.td.gov.hk/FileManager/TC/Content_178/c%20booklet.pdf and see see.

超速是可以書面認罪及交罸款，但傳你上庭就一定與p牌有關，看來很大機會要延長p牌。書面認罪，你連求情機會都冇。

"比警察捉正" the chance to win in this case is low. So, I would say 書面認罪 would better for you.
You can say you are testing the bike/ car before you buy from the owner. and you are not try to commit not to display the P plate.