Yep, Royal Mail are allowed to deliver these political flyers. In addition to the rest of all the mail that has to be delivered. Whether or not your postie supports the leaflets that he is delivering to you, he has to deliver it. Between now and the election he will have to deliver leaflets from virtually every political party.
BTW, i'm a postie myself, and have been since leaving the navy 11 years ago. I have to deliver leaflets of parties that I don't agree with, but I have no choice in the matter. I don't like for a minute, having to deliver unsavoury BNP leaflets, no more than most people like to get them, because I am definitely NO racist, but I have no choice in the matter. Bear in mind that in any delivery office, there will be a scenario where mixed race posties will have to deliver this rubbish, so please have some sympathy for the posties who have to deliver this stuff...it's not our fault, hope this has explained it for you