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It’s pretty weird. Reading through
https://www.gov.uk/guidance/check-if-you-need-to-pay-tax-when-you-sell-cryptoassets#report-pay, you are responsable for making the call by either:
How to report and pay
If you need to report and pay Capital Gains Tax, you can either:
• complete a Self Assessment tax return at the end of the tax year
• use the Capital Gains Tax real time service to report it straight away
The amount of tax due might be different if you are not a resident in the UK.
If you complete a tax return, you must complete it in pound sterling.
Blochchain.com states the following on taxes (see
https://www.blockchain.com/legal/terms):
14.2 Taxes
It is your responsibility to determine what, if any, taxes apply due to your use of Blockchain.com Services, and it is your responsibility to report and remit the correct tax to the appropriate tax authority. Blockchain.com is not responsible for determining whether taxes apply to your Virtual Currency transactions or for collecting, reporting, withholding or remitting any taxes arising from any virtual currency transactions.
Do post, on this thread, the exact URL at which you are getting the message prompting you to make an upfront tax payment. Sounds scammy.