Whether or not it’s fixed shouldn’t matter at all, just do the math.
Yeah your fixed tariff might be slightly cheaper than an Octopus EV tariff if you exclude the car – but do the math assuming your average mileage in the car.
I will say that we bought our charger ourselves, because we wanted to futureproof our parking bay (no free charger with PHEV) – imho worth it.
I certainly wouldn’t ditch the car just because your wife wants to stay on a certain tariff, that just makes zero sense. Go octopus, if your wife is stubborn for no apparent reason, look into another charger (they can be had used for reasonable money, btw).
edit: around £300 gets you an Easee One (what we bought new). Yeah it sucks a bit, but you make those £300 back within the first 5000 miles easily, assuming you get an EV tariff.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.