I can sympathise - although I didn't actually have the op in the end (due to kissing lesions) - I had a nightmare getting my then insurance company (PruHealth) to agree (this was with Mr Skinner, same team).
They finally agreed to it as the surgeon wrote to them explaining that the operation was part of a trial protocol - which was the NICE viewpoint then. I understood that BUPA were far more understanding - times must be tough, and taking a more recalcitrant stance (very costly procedure, £10k+)...re-reading your post, do you actually mean trial not trail?
Talk to Prof Briggs' secretary - will have seen all this before - there are ways around it, or can you not get onto the NHS list - this is certainly done on the NHS (or was in 2010)? What does he say about the microfracture - if he is recommending ACI which is a far bigger op with a much tougher rehab, there must be a reason - have you already had a failed mfx, or is it a big defect?
Maybe there is new data affecting the NICE situation - look at NICE's website, search for their documentation on ACI, cartilage transplant and similar key words.
Your other option could be Cartifill via Mr Shetty in Kent - however, again, this really is experimental, and again BUPA may well say no
Good luck - it really is a pain - you have private HC and still cann't get the treatment
I am about to try a course of 3 Synvisc jabs to calm my knee down again and may well have to pay despite having BUPA through work