I use the Ian Allan book - British Rail Mainline Gradient profiles. In this it shows a short section of 1 in 490 uphill - becoming level and then uphill just north of Northumberland Park 1 in 864 to a point just north of the old angel Road station and level again for about 1.5 miles.
Enfield lock Northbound and Waltham Cross southbound are also level for a mile. Waterbeach southbound is also level
For 357,s the level stretch is leigh on Sea towards Benfleet. and Barking towards Upminster.
Going back to one of the original posts about 465 from Teynham - according to the gradient profile book that is 1 in 200 downhill increasing to 1 in 140.
What we try to do in those situations is balance a downhill time on a 1 in 200 with an uphill time on a similar gradient.
The video app i used in the Class 379 video was a Garmin Virb Ultra 30 camera. The camera records GPS data and the two are synchronised using a Garmin video editing app on a PC
A better solution is Harrys Lap timer app for Android - combined with a Dual XGPS Bluetooth GPS receiver. The app will record the video and GPS together. This is processed on the phone. No PC needed. And the XGPS device provides a better reception than the Garmin GPS chip.
The XGPS is a more accurate and dynamic GPS device than most phone GPS chipsets too - providing a reading every 0.1 seconds.
That was the combination I used to record the HST video here: