I seem to remember something like 25,000 volts are used to test insulation. It is referred to as pressure testing and it is a standard test done on supplies to road lights mainly after road works to see if workers have damaged the outer installation during the works.
However this is not a DIY test and if you want more information I would post on a professional forum.
I know someone you had this job in Birmingham area but never enquired to exactly what he used and what were pass and fail limits.
The insulation tester that i use has 500v or 1000v settings. The results must be above 100 meg ohms for plastic insulated (XLPE, PVC) cables and above 10 meg ohms for paper insulated cables. We use pressure testing on HV cables/joints. It is done at 18KV DC.