If using Python then then you can use the ‘Postcodes’ python library to convert postcode to lat/long and then matched those against the site-lat/long pairs. https://pypi.python.org/pypi/Postcodes/0.1
Looking at the source code for that library, it’s just making API calls to this site: http://www.uk-postcodes.com/api so you could alway go to this page direct. The page also lists a couple of other libraries that use it’s data.
I hope this helps
Clare