LONDON—Transparency advocates are calling on Britain to tighten the country’s defenses against money laundering and tax avoidance after a massive leak of financial data showed how London is a key destination of choice for some of the world’s richest and most powerful people to conceal their cash. The cache of almost 12 million files shows how wealthy people around the world reportedly set up offshore companies to buy the property and avoid taxes. Foreign individuals identified as beneficiaries of these types of offshore accounts in London include Jordanian King Abdullah II, Azerbaijan’s President Ilham Aliyev, and associates of Pakistani Prime Minister Imran Khan. Abdullah has denied any impropriety and Khan tweeted that his government would investigate anyone mentioned and take appropriate action if wrongdoing is found. Aliyev hasn’t commented. The leaked financial data, dubbed the “Pandora Papers,” was published Sunday by the International Consortium of Investigative Journalists and its media …