There is a useful website that helps convert Grid References to Postcodes. See www.gridreferencefinder.com to go from Post Code to Grid Reference, and to a certain extent Grid Reference to Post Code. Also useful for Latitude and Longitude.
There are facilities that give you Grid Reference direct from Streetmap.co.uk. On the main map page, at the very bottom, you will see a link that says “Click here to convert coordinates.” Clicking there will give you the Grid Reference of the arrow, on a new page.
If the arrow is not pointing to the correct place , there is also a button that allows you to move it on the main map page.