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Atmospheric pressure depends on the weight of the air above your head - so the higher you go, the less air there is above you, so the pressure is less. 

Halesowen is 150 metres above sea level, but so that pressure measurements from different places can be compared with each other, they are all adjusted to show what the air pressure would be if that place were at sea level. This adjustment will make sea level pressure between 16 - 18 mb higher than actually measured:

You know when you drive up or down a big hill, and your ears go funny? It's because of this pressure change, which is 1% for a vertical climb or fall of about 85 metres.

Yes there is a formula for sea level pressure, and it is on an Excel file you can download, 
just right mouse click on this link and Save Target As...

Put your own observed pressure, height above sea level and the temperature into the blue boxes, then the pressure adjusted for sea level will be calculated in the yellow box.  The table above is based on this calculation.