Also, you might get a better deal by going through other sites rather than direct - ie it is cheaper to buy through ThinkBroadband than it is to buy direct from the BT website.
If willing to wait and providers play ball as per usual, then Black Friday deals are often the best but no guarantees, as changes in code of practice/advertising may curtail activity.
I have BT offers set to update, but I don't actually know what to yet. The reward cards vary between £75 and £150 and activation fees are also varying too as is the monthly fee.