As been said wait until your last bill and then as soon as that is paid cancel the direct debit.
the reason why you get charged to cease is because Bt charges, i had to pay £29.90 when I ceased from ADSL24 and they cancelled my DD before I had chance to
Not sure if TT will do that.
Desktop machine now powered by windows 7 pro 64bit , laptop by ubuntu
ALLPAY Wireless broadband