Is the MTD threshold based on turnover or profit?
Turnover — not profit. HMRC uses your “qualifying income”: the total (gross) income from your self-employment and/or property before any expenses are deducted. Someone with £55,000 of turnover but only £20,000 of profit would still be above the first threshold. It's the figure on your Self Assessment return before expenses that matters, so it's worth checking yours.