LONDON — Britain was tense Wednesday as police and anti-racist campaigners, fearing a possible outbreak of the right-wing rioting that erupted across the country in recent days, flooded into several neighborhoods with large immigrant populations. The handful of anti-immigrant demonstrators police encountered in north London neighborhoods like Finchley and Harrow were vastly outnumbered by hundreds of counterprotesters carrying signs that said “Refugees Welcome” and “Racists Out, Refugees In.” And in the east London neighborhood of Walthamstow, an even larger gathering of more than 5,000 anti-right protesters chanted “Love, not hate” while hundreds of police officers kept watch. There were also...