This is not tax advice — talk to an accountant — but the calculator toggles exist for a reason.
VAT on platform fees is common for UK VAT-registered businesses: the marketplace fee often includes VAT you can reclaim, but the cash cost is higher until you reclaim it. Remitting sales VAT removes the VAT slice from what you keep if your prices include VAT.
Know which regime you are in before you trust any take-home number.