Missed the self-assessment deadline by a few days — what penalties will I face?

Yes, it's a flat £100 to start with — that's the initial penalty for filing up to 3 months late, so even being a few days over triggers it, unfortunately. After 3 months it gets worse: £10 per day up to a maximum of £900, then after 6 months a further 5% of the tax due or £300 (whichever is more), and again after 12 months. There are separate penalties and interest for paying the tax late too, so the filing and payment sides stack up independently. As for appealing, it isn't a waste of time if you genuinely had a reasonable excuse — HMRC does consider appeals on their merits, though a difficult year on its own may not be enough. The best move now is to pay what you owe as soon as possible to stop interest building up, and get this year's return in early.