why did Manchester bother with me??

basically I applied to the foundation engineering course predicted DDD and had a C already, went to interview etc they said the only reason that I made it that far was due to my PS and reference, but gave me a CCC offer :/ now I know I'm never going to make that, I can;t see why they gave me an offer>?

Because they thought you had potential, but at the same time have a minimum standard they need all their students to have achieved, because experience has taught them that students with lower A level grades tend to struggle. If they knew this offer was impossible for you to meet (as in, you weren't taking the right subjects and enough of them) then you should have queried it at the time. If what you mean is that you don't think you've made your grades, that is an extremely common feeling at this stage of the game. Most people turn out to be wrong about it