I meant to follow up on this earlier.
Dazuk - you can get a tube of 110g for around £2.40 from larger Supermarkets. The Free From/Gluten Free aisle in Tesco usually stocks in.
Slimed, I did some experiments with this a few months back - in the end, I figured you need to use around 1% of the powder. So, for arguments sake, 10g of powder would make a good, thick slime when added to 1 litre of water. 110g would be fine for a bucket (somewhere between 8 and 11 litres, depending how thick you want it).
If you're looking at a bath sized amount, you're probably looking at around 100 litres (possibly double, depending how deep you want it) - so about £25 worth of powder. I've not found anywhere to buy it in bulk, but you could pick up a few tubes in advance each time you go into Tesco.
It's a different type of gunge compared to Nastrasol - Xanthan tends to get thinner when poured, then thicken up again, whereas Nastrasol stays thick all the way through.
One final note - use a hand mixer for small quantities, or a drill with a paint stirrer attached for buckets or more, ALWAYS keep stirring, and add the powder in slowly. Otherwise it clumps up and won't thicken correctly.
Phew. Sorry 'bout the long post, but I hope it's of help.