Making a shiny app to check OS Grid references

Recently I was working with the OS Terrain 50 dataset to make a basemap. Its a wonderful resource, but if you download all of the data as gml files you will be faced with a very long list of folders containing all the data you want but organized by OS Grid reference numbers. Not knowing exactly what I wanted of the top of my head I though it would be useful to quickly put together a tool that would allow me to draw bounding boxes and get back a list of all the grids it contained.