LEIGH-ON-SEA, England—Leaders from across the political spectrum came together Saturday to pay their respects to a long-serving British lawmaker who was stabbed to death in what police say was an Islamic terrorist-related attack. His death has reopened questions about the security of lawmakers as they go about their work. The slaying Friday of the 69-year-old Conservative member of Parliament David Amess during a regular meeting with local voters has caused shock and anxiety across Britain’s political spectrum, just five years after Labour Party lawmaker Jo Cox was murdered by a far-right extremist in her small-town constituency. “He was killed doing a job that he loves, serving his own constituents as an elected democratic member and, of course, acts of this are absolutely wrong, and we cannot let that get in the way of our functioning democracy,” British Home Secretary Priti Patel said after she joined others, including Prime Minister Boris …