I quit smoking last year in June after being a smoker for 18 years. I was a smoker when I was diagnosed with UC.
Everytime I tried to quit, I'd get a flare. So basically I made it 4 to 6 weeks without smoking and I would start smoking again because of the flare. The flare indeed disappeared after I took up smoking again. It never prevented future flares though.
Now that I am smoke-free since 16 months, I experience more flares which are severe and long. I am not willing to take up smoking again though. It was very hard for me to quit and just thinking of going through that smoking withdrawal again is keeping me from ever lighting up again. Left alone the costs of smoking, the other health risks (cancer, stroke, heart disease etc.), the smell, yellow teeth and whatnot.
Bottom line is that I have UC and flares even when I smoke, although the flares were kinda mild and short. It is something you have to decide for yourself, but in my opinion the benefits of smoking on UC are minimal and not worth the risk of getting nicotine related diseases.
Just my 2 cents.