It'll be opt-in and this isn't a new thing - there's several UK insurers that'll give you discounts and bonuses for life insurance if you log workouts with their apps. This just makes it easier for both sides.
There are probably enough sensors to detect untreated sleep apnea for example. Or the pattern of arm movement, heartbeat, sitting in one spot, etc, associated with binge drinking. Or early onset of Parkinson's. Or perhaps false positives associated with these things.
I'm not personally willing to create the equivalent of an OBD-II port on my body and let an insurance company have unfettered access to the data. Insurance companies, at least in the US, have shown they aren't shy about finding ways to cut their costs at the expense of someone's well being.
Although you get rewards from them (Starbucks drinks, cinema tickets, etc.) rather than straight discounts on your insurance.
Marketers would have a field day with that data.
"This guy had increased heart rate followed by two minutes of physical activity... Joe, call Pfizer immediately!"
Accelerometers and GPS would allow them to narrow down specific types and patterns of sex as well. Lots of sex in cheap motels == high risk. Correlating customers together..this one has HIV, and appears to be having sex with this customer == high risk. Fun.
This is not about some extra win-win motivation to be healthier...
It's about turning risk (which is supposed to be what insurance companies are paid for) into rent-seeking...