Tim Worstall has an original explanation of Polly Toynbee's support for Harriet Harperson, rather than Hilary Benn:
[N]iece of a Countess supported by g-g grandaughter of an Earl.
Hilary Benn is, of course, merely the second son of a Viscount.
Further, the Harman connection is to a title from 1677, the Toynbee one to a title from 1322, revived in 1660.
The Viscountcy dates only from 1942.
Easy to see what's happening here, the ladies are simply ganging up on some unspeakable arriviste.