In general terms, a reasonable excuse means an excuse that an ordinary, prudent member of the community would accept as reasonable under the circumstances. Failure to comply with something should not simply be a deliberate act of non-compliance. Rule 6 (of the Restrictions Regulations) requires that, during the emergency period, no person may leave the place where they live without a reasonable excuse. So what is a “reasonable excuse”? Regulations frame the meaning of that term.
They do this by providing an open list of reasonable excuses. It's not a closed list because it states what “includes” a reasonable excuse, and then provides a list of 13 of those excuses. But even this isn't a “gift” list of ways to get out of the house. On the contrary, the terms of some of the excuses themselves limit what is reasonable within the meaning of the Restrictions Regulation.
If an authorized person reasonably believes that a person has committed an offence under the Restrictions Regulations, they may impose a fixed fine.