A Customs Protest is a way to correct information previously entered on entry documents. The most common way protests are filed are either electronically or on CBP Form 19. A Protest can be accompanied by additional duties owed to Customs or a duty refund depending on what information is being corrected.
Whilst the FTA (free trade agreement) means that most imports of goods from the EU will not have Customs duty on them, duty could still be payable if the goods originate outside of the EU (or even if they comprise of components from outside the EU). Any Customs duty that is payable is due when the goods enter the UK customs territory unless a duty deferment account is in place and/or a duty relief is claimed.
Yes, personal medicines can be entered with a prescription provided that the medicine is not restricted or prohibited, and the quantity should fit for personal use, and in case of a restriction must consult the competent authority in the country to allow entry.