Finding the QRA of a site
http://k7fry.com/grid/ New site for finding locators to 10 character accuracy by clicking on a map.
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!
A very useful site with "click to see the path" https://www.solwise.co.uk/wireless-elevationtool.html
Software used offline /P and at home by e.g. Brian G4EWJ is Radio Mobile by Roger VE2DBE. Roger's website links to the G3TVU Quick Start guide aimed at UK users which has UK-tailored versions of Radio Mobile. Ian G3TVU has done a thorough job so set aside an evening for building Radio Mobile UK.