What is the likelihood of colitis stopping me from getting a class 1 medical?
It's almost impossible to answer THAT particular question to any useful or meaningful extent at all, because there are just so many unknowns and variables involved.
For example : your age ? type of colitis ? (there are many), how long for ? symptoms produced ? type (s) of treatment required ? (eg surgery, or steroids, or what ?). See what I mean ?
Two practical suggestions :
Firstly, without even moving from your PC you can look up the JAR Aviation Medicine Manual. It's been referred to on here quite recently in another thread regarding Crohn's Disease.
Secondly, and much more useful for an individual case, is to ask your GP to compose a brief resume ( 2-3 paragraphs, no more ) of your case history, and submit it to the CAA Medical Department at Gatwick.
They are extraordinarily helpful in matters such as this, and will advise you on what is likely to be possible, what is not likely to be possible, and, more importantly, exactly what further information or tests are required to enable a firm decision to be made.
Good luck ! !