Definition:Sterling/Penny/New/Linguistic Note

Linguistic Note on New Penny
When the new penny replaced the old penny, a new colloquial language needed to evolve in order to distinguish between old pennies and new pennies.

While referring to old pennies, the pronunciation of the plural pence had evolved to sound something like p'nce or b'nce or even mce, depending on context.

That is, for example, sixpence would come out sounding like six-bnce.

Hence the pronunciation of the plural of new penny would be carefully enunciated as new pence, or just pence.

But this proved too much work, and so to distinguish between, say $2 \mathrm d.$ for the (old) twopence, pronounced (and even sometimes spelled) tuppence, and $2 \mathrm p$ for $2$ new pence, it became customary just to say $2 \mathrm p$, that is, two pee.

And there it remains to this day: to specify the number of (new) pennies in a sum of money, the word pee is used.

Yes, you guessed, since then there has been plenty of ribald humour based on just that pun, but in general usage, the British public barely notice.