What percentage of UK academic faculty are in UCU?
I don't know what the numbers are for academic faculty specifically is, but the numbers for education in general is 47% (https://www.gov.uk/government/statistics/trade-union-statistics-2018), and that is in line with membership within my department (STEM, Russel Group). UCU has a membership of 120,000. 

As well as collective bargaining on pay, conditions and pensions, perhaps the most important work of the Union is local. On a collective level the union will represent its members in situation where your employer might be going through a "restructuring" involving a reduction in headcount or a change in contractual terms and conditions for example. This has happened to me twice in my career, once when my employer laid of 30% of staff in my area, a second time when I was transferred from being employed by UKRI to being employed directly by the university. 

But perhaps the most important thing is casework. Your membership would entitle you to representation in any HR dealing with your employer. In the case of the worst this can include provision of a lawyer or solicitor paid for by the union. This might be important if you are subject to discrimination at work, or accused of discrimination, or placed on performance review or have been otherwise mistreaded. In the five years I've been faculty, I've seen colleagues require union help with probation, and one (a professor) require union help when they were placed on performance review. I also knew someone who had to involve the union when their head of department took a dislike to them and did everything they could to get rid of them. 

One way to think of union membership is like health insurance, but for employment/legal matters.