File the return anyway, on time, and then talk to HMRC about paying. This is the most important thing in this whole list and the one people get wrong most often, because the instinct when you cannot pay is to avoid the whole subject. Filing and paying are separate obligations with separate penalties: not filing costs you £100 immediately and escalates, and it removes any goodwill from the conversation that follows. HMRC operates time-to-pay arrangements, and for straightforward Self Assessment debts under a threshold you can often set one up online without speaking to anyone. Interest still runs, but late payment penalties are avoided if the arrangement is agreed early enough. The businesses that end up in enforcement are rarely the ones that could not pay — they are the ones that went quiet.