Hey, the exact same thing happened to me! I'm not really sure why there was such a mix up to be honest.
With regards to ICAEW vs. ICAS, I really don't think there's any difference in the long run. I've read somewhere that ICAS is slightly front loaded so you do more exams in your 1st year but can't confirm that until we get more info nearer the time.
Which service line etc are you going to be in? I'm going to be in Energy, UK&I Audit. My friend and future London housemate is also in Audit in Products & Services and she is doing ICAEW so I think it depends on your department as to whether you do ICAS or ICAEW.