Marketplaces and payment platforms usually take their fees before they pay you.
Record the full £100 as income, and record the £12 fee as a separate business expense. Don't record only the £88 that reaches your bank.
This keeps your turnover correct. Your turnover is the full value of your sales, not the amount left after fees. The fees are an allowable business expense, so you record them separately to claim them against your tax.
The same method applies any time someone takes a cut before they pay you. This includes a letting agent who deducts fees from your rent.
You need two figures from the platform, not only the amount that reaches your bank:
Find these figures in the platform's own reports.
When the net amount appears in your draft transactions from the bank feed:
The net effect on your bank balance stays correct. The gross sale minus the fee equals the amount that arrived in your bank account.
Some providers transfer multiple sales as a single payout. When this happens, add up all the gross sale values and all the fees for that batch. Then follow the same two steps against the single draft transaction.