My experience is that Amex will _not_ waive fees in the global transfer programme.
Second, they will initially only offer you a charge card, since that does not involve credit. I was able to wrangle an Amex CC after some major to-ing and fro-ing.
Third: if you are thinking of paying for your charges from a US bank account, this is a BAD idea. As gum outlined above, you can perfectly well use your US Amex in the UK (except for a few machines that are only chip and pin, see UK forum for details), and pay a 2-3% foreign exchange charge (not sure of exact figure). This is actually a pretty favourable rate, since the underlying rate will be the bank rate, rather something you could get at a bureau de change.
If you are going to be transferring large sums of money to a UK-based account anyway (e.g. to pay for tuition fees, pay for rent etc.), then it might be more worthwhile to get a UK card, and use this UK account to pay off your card. Otherwise, you will be facing international bank transfer charges, on a monthly basis: not a good idea.
Either way, you can wait until you get to the UK: if you are a student, you should be able to open a student bank account with most major banks upon provision of an official university enrollment document (and unlike US banks, there are no charges for using cheques, opening and operating a basic account etc.).
good luck,
tb