Unfortunately this post on Seesat-L: http://satobs.org/seesat/Mar-2026/0111.html highlights a change to the mission timeline as posted on JPL Horizons. The TLI burn is now scheduled from MET 1d 01:08 for 8 minutes, i.e. April 2 23:32 – 23:40 with the spacecraft still below the UK horizon (based on a launch at April 1 22:24hrs.
Earth shadow entry is at 23:54hrs with the spacecraft at 7 degs elevation from the Sussex coast, so even an observation of a propellant dump is going to be just about impossible. A launch later in the window won’t change the visibility of the TLI burn for the UK and probably moves it into the Earth’s shadow – although perhaps a 1.5/2 hour delay moves any fuel dump back into sunlight?