I based that on [1] which says in "2021-22 Q2" nationwide, 3.4% cancellations of trains were cancelled, 74.3% were within 1 minute of their scheduled arrival time, 90% were within 3 minutes and 98.9% were within 15 minutes.

Which is obviously far from perfect - but good enough that a holidaymaker visiting the UK has a good chance of not encountering any severely delayed trains.

[1] https://dataportal.orr.gov.uk/popular-statistics/how-many-tr...