Travel and subsistence

I've been in employment through a recruitment agency for several months now and was automatically opted into a travel and subsistence scheme. Initially i thought nothing of it as my net pay was slightly better off. I offer more in sacrifice than i actually get back and had always wondered what happened to that little bit every week. I've tried to opt out of it but they insist i'd be better off if i stayed on it and saw it as kind of a threat that if i do opt out then they will let me go and bring in some one else who is willing to opt in to the scheme.

I'm not sure where i stand to be honest and i'm looking for advice on what i can do.