In reply to:
Well, late or not, you should be able to cancel migration even one day before your activation date... that's my opinion.
I'm not disputing that, but you have to appreciate that the activation date is the
latest date by which your service should be operational and so the closer to the activation date the less likely it will be that the migration can be successfully cancelled. This is simply because of the nature of the work involved; it's not as easy as Zen phoning the guy down your local exchange and saying "he/she has changed their mind".
Notice as well that the T&Cs say
at any time before the Service is activated and not
at any time before the activation date. So if your service has been activated, I would guess you're too late.
You still have the right to cancel as you say, all I was commenting on was the fact that you may now have to accept a period where you have no service as Zen attempt to get BT to reverse the migration work. I expect that Zen want your cancellation in writing because somewhere along the line BT will probably charge them for unnecessary work.