Tbh, we're not idiots. Polls have shown that British sympathies lie mostly with Palestinians, even if they do believe Israel has a right to defend itself as an occupational power, the anti-genocide conviction is enormously strong, and quite rightly so. Tens of millions will be on that spectrum of opinion, and for millions of those it is quite enough to prevent them voting, or vote for a less hawkish party.
Meaning the potential nonmuslim predominantly white vote against genocide is enormous.