There was a link to a YouTube video on here somewhere from some spark discussing pricing for a rewire on a reasonable sized property. He had priced it on time & materials but underestimated the time for various reasons, also he gave example of what it would have been doing per-point pricing and it was not massively different.
If it is for a friend/family you can afford to be a little less critical to price it in order to have something for the assessor, so probably worth looking at both approaches. Get a reasonably detailed list of what has to be done in terms of room sizes, fittings, number of circuits, etc, and make up a parts list for the big stuff. Then try to guess the little stuff (grommets, clips, label tape, etc) but remember it often adds up to more than you expect!
Try to guess at the time it will take, but probably it will be estimated in days (and probably wrong) and then see what it comes to.
Then try the same based on, say, £50 per point. See what sounds reasonable and believable.