Every year, thousands of UK crypto investors file incorrect returns — not out of dishonesty, but because the rules are genuinely different from every other country. These five mistakes are the ones HMRC sees most often, and they are all avoidable.
The five mistakes — and why they happen
Mistake 1: Using FIFO instead of HMRC's matching rules
First-in-first-out (FIFO) is the default in the US, Australia, and most other crypto tax jurisdictions. The UK is different. HMRC requires same-day matching first, then 30-day look-ahead matching, then the Section 104 average cost pool — in that strict order. Using FIFO can produce significantly different results: sometimes understating your gain (underpayment risk), sometimes overstating it (paying more than necessary). Most spreadsheet templates and non-UK tools default to FIFO.
Mistake 2: Missing crypto-to-crypto swaps
If you only export your GBP withdrawal history from an exchange, you will miss every crypto-to-crypto trade. Swapping BTC for ETH, USDT for SOL, or any other pair is a taxable disposal of the first asset. If you traded on DEXs or moved between assets regularly, you may have dozens or hundreds of unreported disposals. HMRC does not consider it an excuse that you “never touched GBP.”
Mistake 3: Not reporting losses in loss years
Losses must be actively claimed by completing SA108 in the year they arise. You cannot simply ignore a bad year. An unclaimed loss year is a permanently wasted tax asset — carrying forward losses that could offset future gains requires that you report them first. You have 4 years from the end of the tax year, but waiting means risk of missing the deadline without realising it.
Mistake 4: Treating staking rewards as capital gains
Staking rewards are taxed as income (SA100, “Other income”) at the GBP value on the date received — not as capital gains (SA108). Putting them in the wrong category understates your income tax liability and distorts your CGT figures. The cost basis of the staked tokens for future CGT purposes is set at the income value you declared, not zero.
Mistake 5: Starting your import from the wrong year
The Section 104 pool accumulates from your very first purchase of each asset — not from when you started using a particular tax tool. If you bought BTC on Coinbase in 2017 but only imported transactions from 2021 onwards, your average cost is wrong for every sale made after that point. Incomplete historical data does not just affect old years; it poisons every future calculation that draws on the pool.
How to avoid all five
The simplest fix is to use a tool that applies HMRC's rules correctly by default and imports your complete history across all exchanges and wallets. Polyconomic applies all three matching rules in the correct order, identifies staking income separately from capital disposals, and prompts you to import historical data back to your first transaction.
If you have filed incorrect returns in previous years, HMRC generally treats voluntary disclosure more favourably than waiting to be investigated. A qualified crypto tax adviser can help you quantify and report any discrepancy.
Who is most at risk?
Active traders who swapped frequently between assets, anyone who used DeFi protocols, and people who have been investing since before 2020 are most likely to have errors in their historical returns. If you started on a UK exchange buying with GBP and sold everything for GBP, you are the least likely to have issues — but worth checking.
Frequently asked questions
Can HMRC really tell if I used FIFO instead of the Section 104 pool?
HMRC may not always detect the specific calculation method used, but they can detect inconsistencies between exchange data they hold and your declared figures. If HMRC runs an enquiry and your records do not match their data, the burden is on you to justify your method.
What happens if I missed reporting a loss year?
You have 4 years from the end of the tax year to claim losses. For example, losses in 2021/22 (ending 5 April 2022) must be claimed by 5 April 2026. After that deadline, the losses are permanently forfeit. Amending a return or submitting a late claim is possible via HMRC's self-service or by writing to HMRC.
Does every crypto-to-crypto swap need to be reported even if the gain is tiny?
Yes, technically all disposals must be reported if your total proceeds exceed £50,000 or your net gain exceeds £3,000. However, if both thresholds are missed in a year, you may not need to complete SA108. That said, every disposal still contributes to your Section 104 pool calculation for future years.
I only received small staking rewards — do I still need to report them as income?
There is a £1,000 trading and miscellaneous income allowance. If your total crypto income (staking, airdrops, mining) is below £1,000 in the tax year, you do not need to report it. Above that threshold, the full amount must be reported as income.
My exchange deleted my old trade history — what do I do?
If you genuinely cannot obtain historical records, document your reasonable attempts to obtain them. HMRC expects you to use best efforts. Where exact records are unavailable, a reasonable reconstruction with supporting evidence (email receipts, bank statements showing deposits, blockchain explorer data) is better than ignoring the period entirely.
Is it worth paying an accountant to fix past mistakes or can I do it myself?
For straightforward corrections (a missed loss year, misclassified staking income), amending a self-assessment return yourself is manageable. For multiple years of errors, complex portfolios, or if HMRC has already made contact, a specialist crypto tax accountant is likely worth the cost.
Tool information accurate at time of writing.