Oh really I didn't realise! That's interesting.
Sarah Parnell's announcements are very clear, pleasant and cheerful sounding. She is a very good voice artist. It is only when she fills in for other voice artists that she sounds a bit odd - she clearly believes Emma Clarke to sound rather slow/drawn-out, low-pitched and miserable, and so she affects that for the Central, Bakerloo and Jubilee* lines, clearly thinking they'll fit in more.
Similarly, hardly anyone could tell that the new announcements on the Northern line were done by Elinor Hamilton, because Hamilton was clearly trying to sound like Celia Drummond (again, not the best impression - if I can convince 2 mil. people that it's MY voice you hear on the Northern line, being a Canadian bloke with a man's broken voice, Hamilton can do better

), and normally Hamilton sounds totally different when heard on the platforms - although that isn't her natural voice. Elinor Hamilton speaks with a Northern accent as like my mother she comes from Bolton
If Parnell voices the new tube stock due in 2025-ish, I guarantee it'll be with the friendly, suitable voice heard on Victoria and subsurface lines.
*Yes, I know the announcements replace Celia Drummond here not Emma Clarke, but I hardly think Sarah Parnell will have been filled in on details such as that. She'll have probably been told to do another fill in and therefore just done the same odd Clarke impression that she always has done.