I'm an ATCO and was working several years back when Conc was returning from JFK and on for a new record crossing time. Still out over the ocean, it had been coordinated straight to CPT for straight in 09L (LL actually using 27R at the time for landing!). Wound the radar out and put on the ground speed to find it doing 750kts up the Bristol channel towards Lundy, followed by a dirty dive from FL250ish from east of Lyneham. Seem to remember a KLM I think being switched to 27L from the R as Conc was about 15 dme and still very fast!
Ah, the days of no late night holding!
Nowadays, fastest things seem to be F15's crossing Daventry CTA to and from east anglia, often indicating 600kts or so. And the occasional near supersonic Ryanair coming south over Manch/Brum with a nice tail wind!