Budget food/living

nope, it would still take 6-8 hours to cook something like oxtail, you are waiting for connective tissues to break down,

I did some in around 4 hrs on the stove top the other day, but would have been even better in a slow cooker for 8.

..thus consuming ~150W for 6-8 hours.
If you cook like that on a regular basis, your electricity bill must be pretty high

Nonsense. 150 Watts for 8 hours equates to 1.2 kiloWatt-hours (kWh or "units") . Typically we'll be paying around 15p/kWh for electricity, so that's 18p a time.

If you can afford the water, an onion and turnip from the fields and the occasional scraped-up bit of road-badger, you can afford to slow cook.