As services are not covered by the distant selling regs and time is of the essence to avoid charges I would contact a forum mod dont expect by return response but its some times quicker than dealing direct
However I would still contact BT sales and try to cancel
Include a link to this topic account number and phone number
you could try uk sales on 08007830056
you should be able to cancel provided it has not been installed.
as hamish72 said - time is of the essence - you need to cancel asap
Make sure you keep the mail just in case chances are it is cancelled but the right hand has not told the left
I would email the mods and get them to check and confirm that your infinity order has been cancelled