If flying above the transition level (in the UK generally 3,000 feet amsl) pilots flying in accordance with the Instrument Flight Rules are required, and pilots flying VFR are advised, to use QNE. This is a standard setting of 1013.25 hPa. Aircraft elevation is reported as "flight level".
That's not quite correct. QNE is not the standard altimeter setting, but rather the pressure altitude at a reference point (usually an airport), i.e. the altimeter reading with the standard 1013.25 hPa set on the subscale. See ICAO Doc 8400.