I do not know how active the IUSW is now. The arguments over exactly this issue took the life out of it. Although everyone agreed on most things, some did not agree with the open to all within the industry policy. After winning a meaningless vote at the most toxic meeting I have ever attended, they then proceeded to do nothing.
No doubt the GMB with its 600,000+ members would be delighted to be told that it is wrong for a trades union to be open to everyone in an industry. Unite the Union with its 1,200,000+ members seems to have the same rules about this, and managers are among the 1,300,000+ members of UNISON.