From About.com Genealogy blog comes this interesting posting. I checked the list and Pennington is not one of the top 100 English surnames.
Hereditary surnames, the family names passed down from father to son, came much later to England than many other parts of Europe. It was not until almost the 16th century before surnames were in common, everyday use by almost everyone. What's interesting is that the most common surnames in use in England today (according to ONS data) are not all of English origin. Their are several Arabic names in the top 100, along with name of more expected Welsh or Scottish origins.
Is your name one of the most common in use in England today? Do you know what it may mean or where it derives from? While it is fairly difficult to say with a certainty how your specific family name evolved without tracing it all the way back to its progenitor, you can at least learn a little about the common meaning and origin applied to your surname in this list of Top 100 Common English Surnames & Their Meanings.