Unlike David Davis, they have to be prepared for anything, including the UK walking out with no deal at the end of 2 years. Should that happen we would immediately lose all the benefit of the Customs Union and all imports and exports would have to be checked at the borders.
Please, Ana, could you read some of the stuff we link to instead of just saying 'We don't know what's going to happen'? No. we don't but we need to know what can be expected under different scenarios. And our border agencies would be utterly irresponsible if they had not looked at all secenarios and planned for them. (Though strangely enough, our Brexit Minister can say that he has no plan at all for a 'no deal' scenario and Leavers think he walks on water)