If a METAR is sent from an accredited observer (but not necessarily from an automated source*), the Met Office at Exeter generates a TAF automatically.
First METAR of the day the computer will analyze the result and PREPARE a TAF and when the next one comes in it will be distributed.
*An automated source which does not have a human observer present MAY not be 'approved' for distribution; you'll have check this with the Met Office and likewise if the METAR comes from another company such at Netweather or WeatherPro.