Finding the QRA of a site
The first one appears to no longer work (Dec 2017)
This is a handy site for finding locators to 8 character accuracy. The magnification +/- buttons do not seem to work, so you have to use the mouse wheel.
Alternatively, try this site by F5UKW to find QRA locators from Google maps.
Lat and Long conversion
UK postcode to Lat and Long: https://www.freemaptools.com/convert-uk-postcode-to-lat-lng.htm
Google map based site to find Lat and Long, OS grid reference and address including post code: http://www.gridreferencefinder.com/
OS NGR to Lat and Long
Convert the OS map 10 digit reference such as TG51401317 to Lat and Long here: http://www.movable-type.co.uk/scripts/latlong-gridref.html
Topographic Google map
So exactly where are the highest sites near me? Use this Google map based site overlaid with height / topographic data to find out: http://en-gb.topographic-map.com/
Overlay OS maps
Directory of portable sites Wiki pages describing some sites suitable for ATV and microwave operation
A Google map listing known portable sites - please add yours!