I'd safely say that newplast has got to be the best stuff to use, it's pretty thick stuff, so you'll have to warm it up a bit before you sculpt. As it is so thick however, it won't end up collapsing under the heat of studio lighting, perfect if you're going to be at it for a couple of hours.
As for the wallace and gromit model kit, (I presume you mean the plasticine one as there are some airfix kits too) I believe that some Hobbycraft stores may still have some on the shelves. Because not many places have these anymore, it might be that the price on the kit will have been reduced.