I've noticed there's a slowdown during the day, which is why I usually do all my editing at night. (it's amazing how fast the internet is at about 3 a.m.)
I don't know if it's just limited to Guitar Tricks though.
If you're on a Cable Modem, the speed will depend on how many people in your area are signed on. At least that's the vague recollection I have from the info I received from my IP when I first joined.

I don't know how well this works but you could try comparing your speeds between the U.K. and various places using this link:

http://www.speedtest.net/

From Toronto to San Fransisco this morning I've got 9736 upload and 904 download.
But Australia is like only 1038 upload, which is awful.