Large gender pay disparities persist within the U.K. operations of Wall Street's biggest banks.

Thursday is the deadline for companies with at least 250 U.K. employees to report gender pay gap information to the British government; U.S. investment banks with a presence in the U.K. are required to do so as well. Studies have shown that measuring and publishing diversity metrics drives improvement — and that more diverse companies perform better — and the U.K. data provides a rare insight into the usually opaque pay practices of America's corporate giants.

Of the big four Wall Street banks, Bank of America Merrill Lynch and Wells Fargo were the only two to report improvements in both the median hourly and median bonus pay gaps in 2018.

Bank of America Merrill Lynch reported a median hourly pay gap of 29.2 percent and a median bonus pay gap of 55.1 percent, down from 30.5 percent and 56.4 percent, respectively, in 2017. Wells Fargo reported a median hourly pay gap of 21 percent and a median bonus pay gap of 44 percent, down from 24 percent and 61 percent, respectively.

Most banks continue to say the issue is under-representation of women in senior roles as opposed to lower pay for women for equal roles. The key is to get more senior women into the investment banking, wealth management and financial advising businesses, Bank of America CEO Brian Moynihan told CNBC's "Closing Bell" anchor Wilfred Frost.

"Drive the representation in the places you need it," said Moynihan.

Activist investors and advocacy groups are pressing Wall Street banks to reveal the global median pay gap data, something Citigroup has already done, though a series of shareholder proposals to hit the ballot box at company meetings in 2019.

Moynihan warned against an over-emphasis on median pay gap data, which doesn't take into account gender-based career preferences. These numbers may slide in the wrong direction as banks hire more junior women to improve the pipeline of female employees, say banking insiders.

In the U.K., Citigroup reported an hourly median pay gap of 32 percent (up from 30.1 percent in 2017) and bonus median pay gap of 71.4 percent (up from 67.7 percent in 2017). JP Morgan's median hourly gap narrowed from 26 percent in 2017 to 25.7 percent in 2018, while its median bonus gender gap widened from 41 percent in 2017 to 41.2 percent in 2018. Goldman Sachs' two U.K. banking entities held fairly steady across all metrics except the median bonus pay gap at Goldman Sachs UK SVC, which widened from 30.5 percent in 2017 to 35.8 percent in 2018.

There is much work to be done to improve gender diversity in finance on a global scale. Among America's biggest 100 banks there are just three female CEOs: KeyCorp CEO Beth Mooney, Synchrony Financial CEO Margaret Keane and CIT Group CEO Ellen Alemany, according to research from S&P Global Market Intelligence.

The banks have introduced a number of measures to try to improve gender and ethnic diversity, tying manager pay and promotions to hitting targets, and involving both male and female employees in the process, through training and networking events.