Atmospheric pressure varies over time and space and also varies with height. Since the altitude of the barometer normally stays constant (the station height) a correction is made to the reading to make it equivalent to the mean sea-level reading. This is done so that readings from different locations can be compared, with differences due to height being removed. Aneroid barometers are normally adjusted to mean sea-level values - read the barometer's instructions to see how to adjust the instrument (normally by a screw on the back). Mercury barometers cannot be adjusted (don't tamper with them because a mercury spillage is a health hazard).
To get the value for your barometer, choose a high-pressure day, pressure values are not changing very much - you can watch the TV forecasts for such a day. Go to the Met Office's observation page
and choose the station nearest to your location - on a quiet weather day the distance away from you will not be significant. Adjust the barometer to the station's pressure value. You can check your barometer on other days but will have to compensate for fast-changing pressures or distance if the pressure is low or changing fast. The observation includes information about how the pressure is changing.