I am sorry that you have UC, I had it and it nearly killed me. I ended up having emergency surgery for an ileostomy.
Diet never effected my UC though, so I am just wondering if you find that keeping away from fruit and vegetables helps at all? If it does, then great, at least as far as the UC goes. I know all about weight gain with the steroids too, so I can really sympathise with you.
Is the UC flaring at the moment? If it isn't, then perhaps you could just exercise more rather than trying to diet when you can't eat fruit or vegetables. Just a thought.
Good luck with it all, and one last thing, UC SUCKS!!!