These concrete blocks, originally positioned across the road itself, are a WWII Tank Trap.
In 1940 Cirencester was a designated "Anti-Tank Island", a defensive strongpoint against the threatened German invasion. It was known that the Germans planned to advance north to the A40, skirting Swindon, then swing round to take Oxford. The Home Guard, stationed in the Abbey House, stood ready to man the defences and hold out "at all costs".
I wasn't quite sure before, but these holes in the wall in London Road opposite the entrance to Purley Road are indeed loopholes for guns.....Thank God it never happened!!
For more info:
Entry in ADS Defence of Britain Archive
Peter Grace has all the details in his book Cirencester at War (Amberley Publishing; 2017) - limited preview on Google Books.