<<When an aircraft is taking off it sets it baromoter to the current reading given by ATC and the height of the airport above sea level. Once it has reached this transition level it will then set it to 29.92.>>
<<Barometer>> I believe you mean Altimeter.
<<Sets a reading from ATC and the height above sea level?>> Altimeters only have one setting arrangement so the setting depends on what you want the altimeter to read - sea level, airfield elevation of Flight Level.
<<29.92>> UK uses millibars or hectopascals (hPa) and the standard setting is 1013.2 (same as 29.92 inches)
The Transition Altitude is reached first on climb out and varies around the UK. In major Terminal Areas it is 6000 ft.
HTH.