There are many attributes that make a good 'voice artist' (professional or not) such as intonation, inflection, style (i.e. informative, authoritative), pace but often what sounds good is subjective.
I'd argue it is more likely that a trained voice artist considers and delivers to an approporiate specification for the application. Virtually all the Worldline voices (Anne, Matt, Rob etc) do not deliver these as they are not trained to do so - Matt's railway announcements (as an observation, not a criticism) are an example where you can tell he hasn't been trained as his intonation and inflections are off and he is very nasal in his delivery, regardless how much somebody likes the tone of his voice. Compare that to somebody like Phil Sayer where the intonation and inflections are nearly always spot on.
Personally, I find a trained voiceover artist usually sounds more professional, but sometimes a staff member who has had some announcing experience can be a happy medium if can keep costs down, so it's a weigh up - GWR Rodger and Wessex Trains Ruth both had some announcing experience prior to being 'computerised' which is why their style, inflection and intonation sounded OK in their original recordings, but the longer term continuity is likely to be poor when updates are needed as they may not be looking after their voices as well as a trained professional, who relies on it for an income.
These ones by Grant sound OK and of a similar quality to most non-trained Worldline voices. I think I preferred Neil's personally although they were a bit shouty. I do appreciate when TOCs make an effort in using a professional voice over artist however - e.g. I think Julie Berry or Janine Cooper-Marshall or Trisha Jarman can deliver more warmth or invoke more 'trust' or comfort in their announcements when it is needed (i.e. when things go wrong) than others who just get told to read a script in a studio without any resonance or empathy to their audience.
