Mark,
While I don't reside in the UK or have anything but a cursory knowledge of your insurance, something struck me as odd. You said your AN was 13.6mm currently. It's only grown, by your own admission, 1.3mm in 6 months which is about average. Assuming it grows another 1.3mm (putting it at 14.9mm) in the next 6 months, why would your doctor recommend surgery? At 1.5cm, you would still be within the range of possible radiation treatment.. Presumably, radiation would allow you to only miss work days instead of weeks or months with surgery.
If a concern is how long you would be out of work, then given the size of your AN it seems to me that radiation would be called for as a viable alternative to surgery, which by it's nature would afford you to get back to work sooner.
Keep in mind, I'm not a doctor nor am I familar with your particular case, but I would explore the possibility of radiation treatment given the size..
Just curious...
Regards,
Brian