It's quite easy to mix up the voiceovers, I'm surprised it has taken this long to happen. Basically, all the voiceovers are on one page of buttons on SpotOn and each day the Sound Mixer drags and drops the correct one onto the main page of buttons into a particular slot. When the titles are played from the vision mixer, this triggers SpotOn to play a silent wav file which kicks over to the voiceover on the next button when it finishes. This ensures that the voiceover always plays at the right time.
On this occasion, it seems the wrong voiceover was loaded onto the button. Did Alastair and Charlene present the previous evening, by chance? It may have simply been a case of the Sound Mixer forgetting to assign the correct v/o. This is more likely to happen on the 6.30 programme than any of the others because there is such a tight turnaround from London Tonight. All the other bulletins get plenty of rehearsal time when errors like that would be picked up. But with only the last few minutes of London Tonight being pre-recorded, it gives no time at all to do any serious rehearsals. I'm not providing an excuse, but if you cut costs and staff, you can't expect everything to run smoothly all the time.
I cast my mind back to the rehearsal video I uploaded to YouTube a while back (see 2:00)
I presumed at the time it was previewing the voiceover, but was it a result of Alastair Stewart interrupting the title sequence?