MetaMask Tax UK — How to Calculate CGT on DeFi Activity (2025/26)

MetaMask is the dominant self-custody wallet for UK DeFi users, with tens of thousands of active accounts interacting with Uniswap, Aave, Lido and others every day. Unlike a centralised exchange, MetaMask produces no tax report at all. You have a wallet address, a blockchain, and the burden of translating that activity into HMRC-ready pounds and pence.

Why MetaMask is harder than a CEX

Every interaction with a DeFi contract is a separate on-chain transaction, and many of them have tax consequences that are not obvious. Swapping ETH for USDC on Uniswap is a disposal of ETH and an acquisition of USDC — two tax events in one transaction. Adding liquidity to a Uniswap v3 pool may be a disposal depending on how the protocol handles your deposit. Claiming LP fees is a disposal of the underlying assets at market value. Paying gas is in most cases not a disposable event by itself, but the ETH spent on gas is disposed of at its GBP value when the transaction confirms.

Swaps and DEX trades

Every DEX swap is a crypto-to-crypto disposal. If you swap 1 ETH worth £2,400 for 2,400 USDC, you have disposed of 1 ETH at £2,400 and acquired 2,400 USDC at £1 each. The gain or loss on the ETH depends on your Section 104 pool average cost for that ETH. HMRC is clear that stablecoins are not cash for tax purposes — a trade into USDC is still a disposal.

Liquidity provision

LP positions are contentious. HMRC has not issued definitive guidance on Uniswap v3 concentrated liquidity, but the common UK practice is to treat a deposit as a disposal of the underlying tokens and an acquisition of LP tokens (or a position NFT), with the reverse on withdrawal. Fees earned while the position is active are income at market value when claimed.

Airdrops and governance token rewards

If you qualified for an airdrop because of prior on-chain activity, HMRC will generally treat it as miscellaneous income at the GBP value on the date it became claimable (the snapshot date or, if different, the claim date). This often catches people out — they receive a token worth £2,000, the market moves, they sell later for £500, and they end up owing income tax on £2,000 of income AND potentially a capital loss on the £1,500 drop.

Staking rewards and lending interest

Lido stETH rewards, Aave deposit interest, Compound interest — all income at receipt in GBP terms. When you later dispose of the rewarded tokens, the GBP value at receipt becomes your cost basis for the Section 104 pool.

Gas fees

Gas is paid in ETH. Each gas payment is technically a disposal of a small amount of ETH. In practice HMRC allows allowable transaction costs to be deducted from the gain on the underlying transaction — so if you swap ETH for USDC and spend 0.002 ETH on gas, that 0.002 ETH of gas cost can offset the gain on the swap rather than being treated as a separate disposal. Track it all carefully.

Building a full MetaMask tax report

Paste your MetaMask wallet address into CryptoLens and the calculator fetches every on-chain transaction across Ethereum, Polygon, Arbitrum, Optimism, BSC, Avalanche, Base and Solana, classifies swaps, LP events, airdrops and staking rewards, converts everything to GBP at historical spot, applies Section 104 / same-day / 30-day rules, and produces a PDF suitable for SA108. That replaces hours of manual spreadsheet work.