See a physio/ Orthopod who’ll do some physical tests (probably only basic laxity stuff) and get a fair idea for the issue. Then (or at the same time), really push for an MRI or stump up ~£200 if the waiting list is huge. It is the only way to get a proper diagnosis of the knee trouble.
I would personally rest until I know what is up with the knee. Ploughing on for the sake of it when you are in a lot of pain won’t do anything to help but can only really either do nothing, or do more damage. No-one can advise you which of those two outcomes are most likely without a decent physical assessment and MRI.
I’d rest and think long term and be extremely pro-active in trying to get to the bottom of your pain and be extremely pro-active in receiving the correct treatment/ advice in a timely manner.
IANAD but have been through similar things with knees myself and learned the hard way that waiting over a year for an MRI and trying to carry on regardless wasn’t the optimal thing to be doing