LONDON — The U.K.’s political right wants to quit another European institution. Battle-scarred Remainers are leaving nothing to chance this time. With ministers struggling to stop undocumented migrants arriving on British shores, the poll-topping “Mr. Brexit” Nigel Farage has been pushing hard for another nuclear option: leaving the European Convention on Human Rights altogether. The Conservative Party is jumping on the bandwagon too — and that’s putting Britain’s Labour government, haunted by