The simple answer is that you need specialist equipment or a specialist service provider.  As PK says there are chemical methods for dealing with silver halide film and there were, when ah weer a lad, companies that would do the work for free in exchange for the silver, though I don’t know any of them now.  In one of the organisations I worked for that used a lot of microfilm we had what was called a Volumatic Disintegrator which crushed the film into a fine powder.  It was, however, very labour intensive and unsuitable for the volumes involved and in the end we arranged for our regular destruction contractor to sort out disposal by incineration or extra fine maceration.  This still leaves the problem of disposing of the final product in an environmentally friendly way.

 

There is a US-based National Association for Information Destruction (shredding companies to you and me) which has UK members.  They’re listed here: http://www.naidonline.org/cgi-bin/naid/show_mem.  You could also look at http://www.ironmountain.co.uk/resource/datasheets/datasheet_it.pdf as they claim to dispose of non-paper materials.