What does Airbus means by long product cycles?
A long product cycle means that the lifecycle of a product spans a long time. Aircraft models are often produced for more than 30 years (the 737 and 747 are now approximately 50 years in production). 

At the same time, the capital expenses to build a production line are enormous. And in starting up a production line there are always teething troubles which take time and money to iron out. Therefore it would be a waste of capital to move a production line from the UK to mainland EU. 

That only is viable when a new aircraft model is introduced and an old model is phased out. New models don't appear every year and it seems that most of the current models are still selling well so unless there will be a huge cost associated with keeping the production line in the UK (e.g. taxes, increasing labour cost, unfavourable exchange rate) no changes are to be expected soon.