Yes, you can boil peanuts in a crock pot or other slow cooker. The method for boiling peanuts in a slow cooker is the same as boiling peanuts in a stock pot on the stove.
Rinse your peanuts several times to remove debris and dirt. Fill the cooker with peanuts and fresh water, noting how much water you use. Leave about 1 inch room at the top of the pot. Peanuts will float when first put into the pot, they will naturally sink as they absorb water.
We suggest using about 1/2 cup of salt per gallon of water, which is 8 tablespoons of salt per gallon. We prefer to add salt near the end of the boil rather than the beginning. This seems to speed up the softening of the peanuts.
Be sure to replenish the pot with fresh water as it cooks. I imagine you will be boiling dried raw peanuts, as I'm not sure of the availability of green raw peanuts in the UK.
It will take 20 hours or more of crock pot cooking to soften dried raw peanuts. If you have green raw peanuts then the cooking time will be about 2 hours, more or less, to reach the softness you desire.
Sample peanuts as they boil to test for softness. Some people like them with a bit of snap, others like them softer. Just don't cook them until they are mushy and are hard to remove from the shells.
When the peanuts are nearing the softness that you like, add the measured amount of salt for the amount of water you used to initially fill the slow cooker.
Boiling time determines softness, soaking time determines wetness and salt saturation. Don't be tempted to add more salt until you have allowed the peanuts to soak in the cooling brine for 1 to 2 hours after they have softened. The peanuts cannot fully absorb the salt flavor until they first soften and become hydrated.
When you use the correct amount of salt you can soak the peanuts in the brine for many hours without them becoming over salty.