If you do a search on the internet you can find places that do reasonably priced helium tanks, eg www.balloon.co.uk.
KS 1-2 Helium balloons can be added together to see how much weight they can hold.
Can you pick up a small child? This would give a sense of scale. If you cannot get enough balloons then try to scale down. You could also go out onto a playing field and stand students at either end of the ship's length and width. Take some long string or rope and cross it over in the middle, trying it at various intervals, creating a fishbone effect.
KS3) Make some halving joint structures which resemble the shape of an airship.
Make these out of foam board to demonstrate the light structure. Helium balloons could be used to lift it off the ground!
KS4-5) Design and make a motorised balloon using very small motors which you can get from technology suppliers. Use all lightweight materials such as foam board.