There are two different issues here and you shouldn't mix them up.
You applied for a job. You were told verbally that you were going to get it, by someone who did not have the authority to do that, and you didn't get the job. That is rotten luck and I am not surprised that you feel bad about it, but if the company advertised the job and had a fair process for interviewing and selecting the candidate, then you were not treated unfairly by the company.
Your last line hints that your job description has been changed as a result. If your job continues as it always was, with a different boss then you have not been treated unfairly.
If you feel that you have been effectively demoted, then maybe you have been treated unfairly. But, it is a very risky strategy for you to hand in your notice and then accuse them of constructive dismissal. I am not a HR expert and any advice you
get on the internet should be treated very cautiously.
If you do feel that you have been effectively demoted, my advice is to make a
proper formal complaint which is based on the facts of the job you were doing and the
job you are now being asked to do. Stay calm and factual and avoid sounding like this is just dissapointment that you did not get the job.
If you are serious about risking your income by resigning and taking a constructive dismissal case, I would strongly advice that you get proper legal advice from an expert. Worse case, you end up with no job, no compo and a legal bill.
I have read some newspaper reports of constructive dismissal cases and it doesn't sound simple to prove, espcecially if you do not go through the company HR grievance procedures.