Whilst I can't help with your query, I got an email today from topcashback.co.uk and there are some rather large cashback offers available on various packages. It's worth taking a look before going direct go bt.com
My one off fees (postage for the hh5) was added to the first bill. Whilst shopping around I saw a n ISP wanting to charge over £80 in one off charges and that's without any sweetener to get cash back or shopping vouchers.
My internet providers
BT Dial Up 56k
BT Infinity 2