The best route for high speed would be the lgv route it's built for it.INo train i've used at top speed has ever derail on it.I would say the 403/406 ice and the tgv are the fastest both fully capable of 150mph + speeds.Top speed harder to judge because of the route has grades that make it hard to get to top speed and stay there.
Just saw 364km/h (226mph) very briefly on LGV. Another downhill section and I reckon 370 would be achievable.
Class 66 downhill from Standedge to Stalybridge But if it's speed you want... note the closed Reg yet still wheel slipping!