You have to place the request to the medical institution that holds the information you want - so if you're trying to get the MRI, that'd be the hospital.
Hospitals tend to have different processes but it'll usually be on their website, or you can ask PALS.
There is no charge anymore, but they can take up to 40 days to return the information, and longer if it's "reasonable". You'd be incredibly lucky to get it before Monday.
The request can be refused, or information redacted/removed, if the data controller thinks it references someone else or might cause harm for the patient to see it.
Otherwise, you'll usually get telephone notes, lab results, letters, prescription charts and clinical notes... theoretically you can also get X-rays, but I've never actually know anyone to get X-rays included. I've had a number of MRI and CT scans; and have requested my notes from four different hospitals when I've moved house and wanted to make sure the new hospital had the correct info - and I've never had any MRI or CT scans themselves included. You might get clinical notes on them, if anything has been noted yet, but I'd check that would be enough for the private consultation - I've found they typically want to redo the test if they can't see the actual test results.
My DP is desperate. Surely if the MRI was in front of him on Monday he could make a decision there and then.
I expect that is true, but the chances that you'll have it by Monday are really slim, and the chances that you'll ever get access to the actual MRI seem even slimmer.... Google suggests they're not usually included, and that backs up my experience.
Sorry to be the bearer of bad news... I've been through the process expecting similar before, and it was gutting when it took FOREVER and then the actual images I needed weren't included anyway.