Hi Mike F,
I have been told that stl files are not good for machining from. Isn't that based on a mesh of some sort?
Anyways, for your other question "cheap software", what is cheap to you? Maybe we can recommend what would be the best you can get, for your needs and budget.
I have not had time or money to try out many varieties of CADCAM, but for 3d machining, I don't think you would be disappointed if you look at OneCNC XP Mill professional for starters. It is not cheap in the home hobbyist sense, but I think it has as good, or better, ratio of features for your dollar...er.... pound......, compared to anything else. I still rave about it whenever I can
OneCNC has an educational licence program, but I have never been made aware of the cost of it that way. Give Michael a call at Onecnc as see what he has to say.
Check out www.onecnc.net for contact info.
There is also a support contact closer to you in the UK. His name is Peter.