This article tells us that if elected Prime Minister Ed Miliband would be the first Jewish Prime Minister, this is incorrect. Benjamin Disraeli of the Conservative party was elected to be in the 1860's. Though one can argue that Benjamin became Anglican aged 12, but in using that logic we can scrap the idea that Ed Miliband will become the first Jewish Prime Minister as he has stated that he does not believe in God.
Russia: U.S. plan to provide air cover for Syrian rebels undermines Damascus fight on ISIS (Reuters)
from the article: Man who would be U.K.'s first Jewish PM to visit Israel