Chemical engineering just isn't a very popular course, it's difficult and a lot of people don't find it particularly thrilling. The first year's probably a massive step up from what you'll be used to doing for A-levels, I know it was for me, and I got good A-level grades. All my offers were around the BBC or BBB range when I applied a few years ago (for the 2004 entry), even from places like Imperial. However, even though what most people from Manchester, where I ended up, were offered was just under 3 Bs, almost everyone on my course had far better results than that and I think with anything lower the work would have been very hard.