We have created a web application to find your nearest post box. Whilst at University studying for my Bsc in Computing, we had to create a mobile application. We realised that if creating a Javascript app we could easily convert it to a full web browser application. So with a few tweaks to the code, you can see and test it out at this link. Or just type the URL https://postbox.derekknaggs.com
You will need to allow the application to access your geo-location before it will work and be aware this might not work so well with computers connected via ethernet cable and home broadband which use dynamic IP addresses. It works better when using a computer with wireless as its able to use Wifi positioning systems rather than just IP address geo-location. This app works well on mobile phones and tablets, although the tablet really needs to have a GPS built in for it to work reliably. We plan to add features to this app in future and also release it as a mobile app. Comment below if you find this useful, thanks.