I don't know much, but will suggest things to consider.
All of the hire boats I've been on have a stated airdraft on the safe side, I've had a 7' boat taken through with 6'11" shown on the gauge. I don't know where your 6'6" came from but it's not difficult to accurately measure the airdraft of your own boat, you can do it with full and empty tanks, then you'll know the real figure, worth taking the time to do it.
Wroxham Bridge, being arched, means that any height showing on the gauge can only be correct at 1 point, at a certain width if you like. As your boat has a smaller beam than most broads cruisers you could probably get away with a bit less clearance than you think. There is out there, a detailed measurement of the bridge explaining at what width the height gauge relates to, I don't know if I'm allowed to point out where to find it.
Good luck.