Passed over for promotion for someone who doesn’t meet criteria - next steps? [UK]

I’ll try to keep it brief. I’ve been at my company (tech related) for 7 years in a few positions, 2 years in current post.

A senior version of my role was created, for the 3 of us on the “junior” level to apply for. 1 of the 3 I know didn’t apply, leaving myself and 1 colleague to get the job.

Now, this other colleague has been in post for around 8 months. I have had to provide support and training for them over the last few months, to get them up to speed on various software platforms we build. I have done this as our previous line manager had left in February, so I figured to pick up this responsibility to demonstrate my ability to do the senior role.

This unofficial training continued right up to last week when the interviews were held. I also had to support and encourage them on speaking to clients to demonstrate our products - something I do regularly and was a requirement for the promotion available.

They’ve been given the job over me following an interview. I’m incredibly confused how my manager has landed on this decision given the job description states a deep technical knowledge of all our platforms and the ability to manage key customer relationships.

I have a chat with my manager pencilled in for 2 days to discuss my interview feedback - am I looking at this the wrong way and should just begin searching for a new job?