Gef, you very plainly do not understand how Direct Debits work. Aside from your ignorance, if this does actually happen to you, you're covered by the DD guarantee.
A standing order is an instruction which YOU give to the bank to send money somewhere. You tell them the sort code, account number, amount, frequency etc. Usually this is received 3 or 4 working days after it is sent and is a fixed amount (until you intervene and amend it, if ever).
A Direct Debit is a way of giving certain companies permission to take money from your account. (Only companies which subscribe to the DD guarantee can join in). The company controls *everything* : date, amount etc. so with, for example, credit cards you can have a fluctuating amount (minimum payment). The same time lag is involved but if the company agrees to take it from you on the 4th, they'll process their claim 3 or 4 working days earlier so that this request reaches your account on or after the 4th.
If you have a DD due out tomorrow and call the company to cancel/amend it, they won't be able to as their claim will be in the pipeline.
If you cancel a DD with your bank, you only cancel the "method of payment", not the entire contract between you and the company. If the DD was with someone as reputable as, say, AOL then I would expect them to set up a new DD to continue the payments unless you speak to them and cancel the subscription/contract.
rant over.