Add GPS proximity search (UK ONLY) to existing Wordpress directory website

This is my clients website [login to view URL] Created with Wordpress, It is a directory based website that uses this WP plugin for the directory: [login to view URL] The directory already uses Google maps. I just need a GPS proximity search feature that will allow the following: •User will enter either a town, city or postcode (zipcode) located in the UK (the website is ONLY UK based) into the search bar on the homepage or other pages (link: [login to view URL]) and then click search. •A proximity search of within 15 miles (it will always be 15 miles) will then be performed. •The user will be presented with the search results page (link: [login to view URL]) listing all results within the 15 mile radius of the location they entered into the search bar.