Why mri? Dislocated first rib is result of torn ligament and I have neurological issues now. Gp said that it would have diagnosed it earlier!

Not sure of ? I'm unsure of what you are asking. If you are having neurological deficits thought to be related to a first rib issue, MRI would help to confirm this. It would also be helpful in terms of planning treatment. First rib dislocations are very rare and are usually from very high energy traumatic events.
A must. 1st rib injuries are severe, causing potentially neuro and vascular injuries. Best way to diagnose will be with mri. I agree with your gp.