I used to live on one of the roads on that course many years ago. Rented a house where you see the steps and railings on Crossgate (where the riders are descending before the sharp right turn into the cobbled ascent) next door to The Angel pub.
I've cycled all of that loop in the past, but mostly in the opposite direction (owing to the descent and ascent they they have on the loop being one-way streets in the opposite direction to the one they used). Several laps going up the cobbles of South St. must've been pretty hard going!
Looked like great fun to watch - I might have to try and get to the Colchester one if I am free that day.