The most recent analysis of trade union membership comes from 2015. People in trade unions earn more money than people who are not in trade unions. The longer you have been in service, and the older you are, the most likely you are to be in a trade union. People in the public sector are much more likely to be in a trade union, and people in the north are more likely to be in a union than people in the south east and in London. Very low earners and very high earners are much less likely to be in a union. Temporary staff are half as likely to be in a union. Women and disabled people are more likely to be in a union.
https://www.gov.uk/government/uploa...on_Membership_2015_-_Statistical_Bulletin.pdf
It's interesting seeing which sectors see high union membership and which sectors see low membership. Accommodation and food service sees membership at about 2%. This is probably the most volatile of all the sectors, with the use of zero hours contracts, repeated outsourcing and subcontracting, and immigrant labour the highest.
People in volatile and short-term employment won't be in a union, because they can't afford to be and because their employers won't recognise them even if they are.
People in high value employment won't be in a union because they won't need to be, or because they're in senior management positions where it would be much more difficult to be unionised.
Younger people won't be in a union because they take the rules for granted, and are less likely to be treated badly in employment.