Your 140k is 40/140 gains. So if you withdraw 100k the taxable gains will be 100k x 40/140 = 28571.
The tax free allowance is 12300 so you will owe tax on 28571 - 12300 = 16271.
If there are no further gains and you withdraw a subsequent 20k then that will contain gains of 20k x 40/140 = 5714.
In fact it is a bit more complicated because it depends what you sold to withdraw the 100k. The above calculation is only correct if everything was the same stock, eg you only bought and sold Tesla. Otherwise you will need to calculate gains per instrument. Of course there is no tax to pay on withdrawal of cash that was never invested. If any dividends have been received they are taxed under other rules.
Self reporting of tax liability in the UK is on an honour system, with checks and audits by HMRC and its computers, but with severe penalties if false reporting is discovered. Trading 212 may also be reporting details of client account balances to HMRC. A client’s bank will know that the money went to and from a brokerage account. It is not uncommon for HMRC to write to someone saying, “we think you have undeclared income.”
Accountants and financial institutions are under an obligation to submit a Suspicious Activity Report to the authorities if they have any reason to think a client is engaging in money laundering or tax evasion.