There is lots of useful technical information in the VO Service Guide here:
https://virginorbit.com/wp-content/uploads/2020/09/LauncherOne-Service-Guide-August-2020.pdf
On page 22 there is a launch profile diagram showing altitude (in 1000 ft) vs time. We also have the visibility maps here:
https://virginorbit.com/the-latest/where-can-i-spot-the-launcherone-rocket-on-launch-day/
I am assuming these mean that the vehicle is above the horizon from a particular location so they are very optimistic in terms of visibility. For example at 300s after drop the second stage will be burning and the altitude will be around 500,000 ft or 150km. At 300s according to the launch track it will be around 1000km great circle distance from Salisbury (off the northwest coast of Spain) so a bit of trigonometry gives a height above your horizon of 4 degrees I think.
You might want to check that maths. It is a bit early in the morning…