There are two systematic errors at play there, firstly GMT is the average (hence the "mean") local time as obserbed at Greenwich. Since the majority of the country is to the west of Greenwich it follows that locally observed solar time trails GMT.
The second is that a day is not of constant length. Leave aside the natural variation in the speed of the Earth's rotation due to ocean and atmospheric currents and tidal effect, those are quite small compared that caused by the fact the Earth's orbit is an ellipse rather than a circle. As a result geometery changes (how many times the Earth must rotate for the Sun to return to its highest point) and it also moves faster around the Sun when it is closer to it (in December).
For those reasons sunrise and sunset would never perfectly centre themselves around noon. It's also why if you count the days between the spring and autumn equinoxes you find that "winter" (in the northern hemisphere) is actually several days shorter than "summer".