It's not a conflict of interest.
Leonardo pay a membership fee to Make UK who are a trade association. Make UK represent their members in lobbying and provide group services to their members who may also pay a fee for training courses and materials (as an example).
Make UK will listen to their members and follow their requirements. The board directors and other committees are made of of their members. Staff will advise the board and committees. The whole point of a trade association is for companies to get together and agree collectively what they want and pay the trade association staff to do it for them. There is no requirement for Leonardo or its staff to be independent from Make UK.
I don't believe that the trans issue ever came up at the committee and if it did, they would have asked the Make UK staff to research it and come back with advice. There would be no reason for member companies to disbelieve the staff unless an individual member representative knew the law and said so.
Had Andy known the law, (which he should have), he could have said that the advice received by Make UK was wrong and ask them to consult an employment lawyer for a definitive response.
What's shocking is that Make UK outsourced that advice from Stonewall (probably) rather than employment lawyers. What is equally shocking is that a massive company like Leonardo relied upon the advice of a trade association who relied on the advice of a pressure group, rather than talk to you know, employment lawyers.