Topbunk
The reason is that the UK hasn't yet ratified the international agreement on units,I think.
The international system has gone completely metric and the sub units like the centimeter based ones or the "atmosphere",Bar and the likes have disappeared.
For us the pressure unit should be the "Pascal" equivalent to a
Newton per square meter,which is a ridiculously low pressure,hence the 100xPascal aka "hecto pascal" hpa.