Voiceovers are too often an easy way out of a narrative conundrum. But I just went back and looked over it again a little more carefully.
As far as I can tell, the only voice over16d0 is at the very end and only a few sentences. It's also not Billy Howle, just a generic voice.
I also noticed Florence definitely plays the violin in at least one scene in Wigmore Hall in London. That should be fun.

I personally wouldn't call the voice-over ''unfortunate''; necessary for the flashbacks, etc. I think Billy Howle (Edward) will do the voice-over. I know the producers and the director were working on the script before production began, and I think McEwan said he added some extra scenes too.

Thanks to tarakate!
http://www.raindance.co.uk/site/picture/upload/image/general/movies/on_chesil_beach.pdf
Just briefly skimming over it I notice there is voice-over, which is unfortunate. It's also a first draft so perhaps it has changed.