Polystyrene, around £5.50 for 25mmx2400mmX1200mm
Grout, about £4-5 for 3-5 kg bag
No nails about £5 but you can get it form wilkinsons for £7 for 2 tubes of it
Spray paint or acrylic paintif you are gonna paint it.
I've just started building my viv and background and i am gonna grout and then prime the background (prime £3.50 from wilkinsons and use Plasti-kote rock effect spray paint, it looks really good and gives a textured finish, £7 from b&q, £6 from wilkinsons.
you can pick up a decent size tub of waterproof pva from wilkinsons for £4-5 to seal it.