Called VFR on top at FL 670 in a Lightning. Boulmer were sharper than normal and came straight back with "Ceasing radar service, you're clear en route, click." Speeds as best I can recall were in the order of Mach 1 and 150 Kts IAS at the time, having zoomed from Mach 1.95 (never did see Mach 2).
------------------------------------------------------------------------------------------------------------
All I can say in my defence Your Honour is that alcohol was involved.