We have higher odds of having non-celiac-gluten-intolerances, and dairy-intolerances than the general population (no guarantee of having those issues by any means). You could still have issues with them, despite a negative celiac test. It's easy enough to skip lactose and gluten for a month as a test and see if it helps you or not. I found no benefit from eliminating those from my diet, and still enjoy dairy and gluten daily.
The majority of gasteroenteroligists will say there is no connection between diet and UC symptoms. Why? Currently there are no diets studied in great depth (large scale, long term studies that are placebo-controlled, results duplicated by peers, etc.) and officially approved for the treatment of UC, and until their are don't expect a gasteroenteroligist to say they help. Anecdotally, some of us here have noticed benefits of various diets. Diets, like everything UC-related is hit-or-miss, and results vary wildly person-to-person. I saw no benefit from experimentation with various diets, others swear by them, and that's just UC wrapped up neatly within a nutshell.
Uniform Charlie had posted a really good review of what's currently known about
IBD/UC and diets, and looks at the scientific research available to date on it. Give it a read, link repeated below.
/www.dietvsdisease.org/ulcerative-colitis-diet/If you are so inclined, you can try one or more of the various diets for UC. They are generally do-no-harm and have a chance of helping. Give the change about
a month, but expect results at the end. Keep it if you get results, otherwise discontinue. Possible it could help, and equally possible it might not help. I'd just create a list of plausible things that could help that you are amiable to try, just in case you need them. UC is for life, so you have lots of time to experiment and see what helps you. Lots of trial-and-error, and many of us have been at that for years...