Greewich Mean Time is mean or average solar time at the Prime Meridian, 0 degrees longitude, in Greenwich, in the south-east of London, England, UK. At noon, when the sun is straight above Greenwich, the clock at the Royal Observatory shows 12.00.
The word 'mean' is used to point out that GMT is an average of all the values recorded throughout a year, due to the specifics of the Earth's movement round its axis and round the Sun (view here an excellent illustration on the specifics of this planetary cycle)
GMT is not just a certain clock time, but something else too. So, what is GMT?
The rest of clocks in the UK will show the same time as the time at the Royal Observatory clock only when Daylight Saving Time is not in use.
BST has a fascinating history.