Just read this, hope it all goes well for ya mate. This sounds alot like the ordeal i had and has since pretty much made me lose all faith in the NHS and especially GP's.
I swear they don't want you in their damn chairs and want you out ASAP.
I was told on 3 different occasions (after knee collapsing) to rest for 2 weeks and i will be fine, the physio won't know really either, i was playing for Romsey at the time and the physio just agreed with the GP.
In the end i demanded to go to see a specialist and when i was there i demanded to have an MRI. It ended up i had ruptured my ACL and the consultant (Mr Fox i believe) told me how lucky i was that i had not damaged the cartialage due my GP getting the diagnosis so spectacularly wrong.
Anyways i had the surgery and all is fine and i am playing regularly again (the hard bit is getting over the mental 'i don't want to get tackled' issue) but it is worth demanding getting it done or else it will just drag on.
KEEP UP the physio though, afterwards i spent about 16 - 18 weeks doing physio and you will have stipulations saying don't bend your knee further then 20 degrees etc but you need to do this or it will ruin your new ligament.
Anyways, all the best good luck, shame you got shafted by our wonderful health service like i did.
If you need any more info PM me.
NB - Sorry about the story lol