You can sell wool to BWMB or Texacloth ---I am pretty sure there is no obligation to register with BWMB , I know plenty of commercial producers that aren't
Commercial producers are obliged to inform the BWMB if they are going to sell their wool elsewhere.
I agree that with a small flock of Shetlands, particularly if not white, the BWMB will not be at all worried if you want to stay outside their scheme. However, they are, in my experience, all very knowledgeable, helpful people, so I would not be frightened to give them a call to discuss.