I think you are confused about the nature of conditional offers. I have some friends on my course at LSE, which requires a 2:1, who had offers requiring a 1st--one of whom was an undergrad at UCL. I even know an Oxford graduate who got a conditional offer of a 1st for a course at SOAS.
Conditional offers are based on your current performance. If you have a 70+% average, you will typically be given an offer of a 1st if you get accepted at a top university. If you have a 68%, they will give you a conditional offer based on a high 2:1. If they don't want you, regardless of your current performance, you will get rejected.
Also, not everyone applies as a third year. Several courses (e.g. Criminology, MBA and a few other MScs at Said) value professional experience as well as academics, meaning that a 2:1 would be taken in consideration alongside other things.
Tell me: have you seen any offers for people who have been out of university for a few years? Or is it all just from your friends who are current students? Have you ever considered that they give 1st offers, allowing a small amount of flexibility for people who graduate with a 68, but that they might outright accept a graduate who has received a firm 68?
I agree that an ex-poly grad with a 1st would stand a better chance of acceptance. But I don't accept than an ex-poly grad applying for a course the explicitly states a 2:1 (rather than "2:1 but most of our accepted students have a 1st"--not all of them say that) can't get accepted with a 2:1.