Occurring throughout the play, the voices are a kind of intermediary between the audience and Christopher’s interior state. They report facts, or echo things from within his auditory environment. Their accent should be unremarkable (probably Estuary) unless the thing they are relaying has some particular information conveyed by accent.
I’m thinking here of 32. SWINDON TRAIN STATION, 35. PLATFORM and 37. TUBE TRAIN
For these, the voiceover site https://www.advoice.co.uk/ should provide a rich selection. Here are a few that I thought might work for you:
When it comes to the train announcements, this video should be helpful:
A plausible backstory could include Swindon of London background, and since so many of the Swindon accents I listened to were indistinguishable from London accents, an Estuary accent seems like a reasonable choice.
Here’s Phil Vickery, a rugby player from Devon, who nevertheless, sounds quite Estuary:
Well we’re in Swindon so this breakdown will be the best place to start.
I like this bloke as a model:
Since this is such a short scene, this might be the place to pull out a stronger West country:
MAN BEHIND COUNTER
I believe we discussed Estuary here.
Ooohh! Let’s go full Cockney here! The phrase “They’ve got, like a train elf!” was made for Cockney.