I remember the "clock cracker" page that was part of the 600s on ITV, don't have the slightest clue what it was for though!
It was a stress-test for receivers, that particular pattern was quite processor intensive so if it could be displayed without causing issues all was good. Processors used to be known as the clock.
They were also a test for the teletext inserters which added the teletext signal to the VBI, which is why you'll find a clock cracker of some form on every service.
For teletext to work correctly the 'dots' in the signal which represent the bits of data needed to be in exactly the right place, or the TV would interpret the bits to be at different places in the data, resulting in incorrectly displayed services.
Each inserter had a clock, which had to be in sync with the video signal and switching bits at exactly the right time in order to keep everything functioning. The block and divide symbols together, in bits, represented all 1's with a single 0 at the end. This combination essentially continually stress tested the inserter clocks, as the continuous stream of 1's made it easier for the clock to loose time (they weren't as precise back then as they are now, and easily lost sync if the data wasn't always changing). It also made it easy for engineers to see if the data stream was out of time with the video sync pulse, as all the characters would change, and what they changed allowed them work out exactly how much it was out of sync.