Regular batter mix is for making a savoury batter, along the lines of Yorkshire puddings and the like, while cake mix is for making cakes (obviously) - I believe the main difference is that one contains sugar and the other doesn't.
A quick check shows that the ingredients of most of the cake mixes seem to include sugar and vegetable oil (as well as a plethora of other additives with exciting names), while the regular batter mix is mostly flour + powdered milk. This may have an impact on the texture of the mix (taste as well, but I'm guessing that's not a primary concern ;) ).
You may need to experiment a little with the mix to get it to a suitable consistency and texture, but experimenting is all part of the fun
Suggestion: Whatever you wind up with, test it to see if it acts like flour and water. (Flour and water will stick badly to hair and is very hard to wash out) If it IS like flour mix, simply add enough cooking oil to make it hair-cling-free. Otherwise, the clean up may take HOURS!
Not completely sure what is available in UK big box stores, but cake batter might work pretty well. Does anyone know if it works better without adding the eggs, or just add the water (perhaps more than the box instructions suggest)?
Flour and water is used as wallpaper paste; I suspect (d'oh) it ain't a good idea.
If you want to avoid the inevitable hair-sticking issues with mixes based on wheat flour, you can add a bunch of cooking oil to the mix. That pretty much eliminates it, but makes cleanup a little more difficult.
You can also use corn starch (a.k.a. corn flour) instead. It doesn't have the sticking problem and it creates a non-Newtonian fluid, which has some unusual properties. If you boil it for awhile, it becomes slime or pudding, depending on how much you use.
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Bit late to the party, just confirming that it's cake mix/sponge mix you want, and also to add a drizzle of cooking oil. It helps it wash off, but more importantly gives it a sexy shine.
A bucket is somewhere between 20-30 packets depending on how thick and smothery you want it. You're talking about eight quid a bucket according to my calculator.
To colour it, you want gel colouring if you've got cash to splash, and poster paint if you're on a budget. Regular food colouring doesn't go very far at all. I found this out when I tried to make up something close to blood recently and it ended up looking like Windolene. Good job we had some poster paint in the house, really! Not great tasting, mind.