You've got 3 other choices, all cheaper than the Impossible Project.
Fuji 100 Pack Film - FP-100C, FP-3000B
This is Fuji's version of Polaroid's type 100 pack film, and works with a wide variety of the old Polaroid Land Cameras. A massive list of them is
here, you want a model with "100" next to it. The second hand prices of the Land Cameras varies wildly, some of them were very plastic and consumer orientated, some of them were very serious cameras (the Polaroid 180 springs to mind, one went on eBay recently for £200+). Fun cameras, quite large (but that's generally the case with most Polaroid Land Cameras - and fair enough given the size of the image they make), most use a rangefinder to focus, some use old 4.5V alkaline batteries that are no longer available. Fuji does a colour ISO100 film (silk or glossy - most land cameras had an ISO speed of 75 rather than 100), and a black & white ISO3000 film (most land cameras have an ISO speed of 3000).
£1.26 per shot at current prices
Fuji Instax Wide
Fuji's own take on instant film. The supporting cameras are the Instax 100, 200, 210 and the rarer 500AF. Produce a big image, cameras support modern batteries, cameras are rather large and ugly, the film is a bit cheaper than the Fuji pack film (not by much though). One speed of film available, ISO800. Cameras offer relatively poor control of flash exposure (it's usually on, regardless of what you tell the camera to do). Cameras, second-hand, are relatively cheap - and you can still pick up a 210 new for something like ~£60.
67p per shot at current prices
Fuji Instax Mini
The other Fuji Instax film, smaller cameras and smaller images. Models include the mini 7, 8, 25, 50S amongst others. Portrait orientation. The film is ever so slightly cheaper than the Instax Wide film, although considering how much smaller it is, not necessarily worth it. One speed of film like the wide films, ISO800. Cameras are more expensive on the used market than the Instax Wide cameras.
63p per shot at current prices