Hi parabellum,
Your formula is correct for dynamic aquaplaning when water can't be dispersed fast enough by the tyre tread - but the incident at Christ Church was due to Viscous Hydroplaning (mentioned in the final report).
There is some good info at
Dynamic Vs Viscous Hydroplaning
"Viscous Hydroplaning
This one can fool you, because it can happen even at very low speed. Essentially, it is sliding on some liquid other than water or in a situation where water has mixed with something. For example, an area of a ramp or runway could become contaminated with a number of substances.
The run-up area would be a good candidate here, because a lot of aircraft sit over the same spot for a few minutes. Although not leaking much oil individually, the collective effect of a drop or two from dozens of aircraft can produce a noticeable stain. Then along comes a little rain to lift this out of the pavement surface, and it gets slick in a real hurry.
Touchdown zones are even worse, because they get oil shaken loose by the landing impact and a lot of rubber dust, too. At some really busy airports, they have to periodically go out and grind this stuff back down to the pavement.
Once again, just add water for a nice, slippery mess! In most cases, a good hard rain will wash away most of this, so most occurrences are as the rain starts or after a light shower or heavy dew."
You've probably notice this effect when you turn onto a damp runway in the touch down zone and get that nose wheel skid sensation / noise.