In considering amendments to the current cash controls undertaken by HMRC, the House of Commons EU Scrutiny Committee posed the question of what will happen after (if?) the UK leaves the EU. Currently, declarations are required where €10,000 or more is sent or taken to or from a place outside the EU (which includes to or from the Crown Dependencies). The immediate impact of the UK’s withdrawal from the EU, the Committee notes, will be that the remaining Member States will have to apply the Cash Control Regulation to movements of cash into the EU from the UK and vice versa, unless an agreement to the contrary is concluded before March 2019.
https://publications.parliament.uk/pa/cm201719/cmselect/cmeuleg/301-iii/30136.htm