They can apply before trading begins, but not before the enterprise is real. The lease, equipment, licences and committed expenditure need to be in place, because the requirement is a genuine active undertaking rather than a plan for one.
Set it up first, then apply
Hypothetical example: a Hampton couple ask whether they can obtain the status first and then spend the money, which is the order any prudent person would prefer. The category does not allow it. What is examined is whether a real and operating commercial enterprise exists and whether the investor's funds are irrevocably committed to it, and both are assessed on the facts at the time of the application.
In practice that means signing the lease, buying and installing the machines, obtaining the licences and insurance, and opening the accounts before applying, with the doors opening shortly afterwards. The risk that creates is genuine and should be planned for: decide in advance what the couple would do with the premises and equipment if the application were refused, and take advice on the strength of the case before the largest commitments are made.