maxsteam: If you are sure that you are in the right and as long as you are willing to stand up for yourself, you should cancel any direct debit.
I think we have had this debate before, and I note you have qualified your advice without explaining what might be necessary to "stand up for yourself", but in the absence of agreement about the contractual position, cancelling the direct debit simply invites a credit file default entry and potentially debt recovery enforcement.
I once had a similar though not identical experience with the same company. I fought it on strict interpretation of the contract, contract confirmation timings, the Telecoms Licence General Conditions and threat of reference to Plusnet's Alternate Dispute Resolution provider. They conceded.
One thing that the OP needs to consider carefully is that if the contract confirmation stated an earlier start date for the activation of the service than "7 working days" then the entitlement to cancel ends on the activation of the service. Of course I invite keener legal eagles to confirm or correct that.
The best way (essentially the lowest cost way) forward requires consideration of the reasons why the OP wanted to cancel and whether any alternative service is sufficiently better and/or cheaper to justify the hassle.