You could use a few different free websites to do it.
Walk, run, jog or map my run still have the pre gps function where by you can plot your route manually onto a map.
I haven't used these sites since smart phone app's started to plot your route as you go but I used to plot my routes in when I got home to see how far I had run in a given time.
They might even make it possible to save each plotted leg to give you the whole picture.
I hope that helps, feel free to pop in for a virtual cuppa as you go past London.