[vidarh]

I think any union covering more "white collar" type jobs probably would want to remain unaffiliated. Some also do for other reasons - e.g. Prospect [1], which is for engineers, managers and scientists. Their main reason has been that they organise a lot of people in the civil service and other parts of government, and e.g. BECTU (the broadcasting union) disaffiliated from Labour when it merged with Prospect for that reason, but it's likely that parts of Prospect's membership would also be more politically diverse than many other unions.

[1] https://en.wikipedia.org/wiki/Prospect_(trade_union)