If the US company pays a $1 dividend then $0.85 will appear in your Trading 212 account. So $0.15 has been deducted. Depending on your UK income you will have further tax to pay via your self assessment return. It can be 0, 0.325-0.15 or 0.381-0.15. You take a credit for the US tax withheld.
you used to need to fill it out yourself and submit, but it is now settled automatically by the system when you purchase a US stock and are set to receive a dividend. you should see an in-platform popup that requests you to check and confirm your details.
newer accounts would seem to have the W8BEN made as part of their account setup so that its ready in case they acquire US stocks.
additionally, W8BEN is a yearly form so the system would likely display a yearly prompt for confirmation of details so you are always accurate and uptodate.