Is 2.5% realistic to pay on defence?

The mistake the present government has made, as the previous one did, is to behave as if a particular level of expenditure is desirable in itself. That is always the wrong approach, whether in defence or any other context. First, you need to define what you want to achieve; second, try to calculate how much that would cost; third, decide whether you can afford it and if so how. I have heard nothing from the Labour Party on the first of these, and I don't expect to hear anything soon.