DUBLIN — British Prime Minister David Cameron faced mounting pressure Friday over his involvement in offshore investments after he finally admitted he benefited from his father’s Panama-registered trust, an entity exposed by this week’s massive data leak at a law firm specializing in global tax avoidance. Lawmakers from opposition parties demanded that Cameron tell the House of Commons whether he holds, or held, other similar investments in trusts beyond the reach of the British tax system. The issue — which has ensnared scores of leaders, stars and others since Sunday’s unprecedented leak of 11.5 million documents from clients of law firm Mossack Fonseca — is causing particular damage to Cameron, who for years has campaigned prominently for a crackdown on global tax avoidance. After three days of obfuscating statements, Cameron admitted Thursday night he had sold more than about $48,450 at the time in shares in his late father’s Blairmore Holdings, an investment vehicle that Mossack Fonseca helped organize in the Bahamas.