Just chatting to Vodafone and have done some costings, and it is surprising. It is not quite comparing apples with apples because it is based on an iphone 7 128gb v iphone 8 64gb - this is simply because a 32gb 7 would be pointless so I am comparing the two options I would choose between (would prefer more but I've had my 64gb 6 for 3 years and still have 19gb available so unsure I'd need more). Results as follows:
Iphone 7 - £53.41/month over 24 months
Iphone 8 - £49.18/month over 24 months.
Whilst I do not use more than 46gb now, I wonder if over the next 2 years similar apps and iOS updates will require much more storage. I feel that 64gb is fine but wouldn't want to be caught out after a year needing more.