Sir Robert came from south Staffordshire, was MP for Tamworth, and one of the country's better Prime Ministers. I have a soft spot for him, as, in 1846, fighting his own supporters, he pushed through the repeal of the notorious Corn Laws, which had caused so much misery.
But why Peter Blake (the artist who created the cover) and/or the Beatles themselves picked Sir Robert to be one of their crowd, I do not know. Nor can I find out. If you know, please let me know too!
Sir Robert is the black-and-white figure, two in from the left on the second row down. Just underneath the infamous magus Aleister Crowley.