Coincidentally, I found myself in Greenwich today and, as pictured, stood on the Prime Meridian. These days, as well as a brass strip showing the line of 0 degrees longitude, there is also a green laser beam that shines into the night sky towards the O2 dome, which is visible for about 15 miles.
The Royal Observatory at Greenwich is well worth a visit and of particular interest are the time pieces on display of John Harrison, who solved the longitude problem (the basis of the Channel 4 drama "Longitude" in 2000).
"Oxford time" is 5 minutes later than GMT. I gather that the Great Tom Bell at Christ Church College is rung at 9:05 pm (9pm on Oxford time) every evening for this reason. However, I have to admit that I have never witnessed and confirmed this myself.