The security of supply standards are conservative. Good for reliability, but this comes at a price. This report goes in to more detail: https://www.dcode.org.uk/assets/uploads/IC_Report_exec_summa...
~20 years ago, the regulator introduced an incentive scheme to reduce customer interruptions and minutes lost. This resulted in heavy investment in network automation in the UK.
Can you say a bit more about built in slack? You mean like Distribution Automation switches to backfeed an area? This has felt sort of rare to me