It was once very easy when we were part of the EU. Since then its a pain in the backside. I ordered a barrel to friendly RFD and it was part of a consignment of a good number of barrels. Cost of doing one barrel is prohibitive. The real embuggerance was British Customs who couldn’t work which coloured ink to use to complete the import process so it put on the too bloody difficult pile. Took over 6 months for them to come through and the end cost was nearly twice the cost of the barrel in Germany. The galling bit was UK Customs charged storage fees whilst they worked out how to process things.
Unless the German is particularly special, if I were you I would look at a barrel from one of the UK barrel-smiths, or use a barrel blank that is already in the UK.