It is nothing to do with tariffs. The U.K. distribution for shimano is and always has been in Milton Keynes.
It previously went to Milton Keynes (EU). It now goes to Milton Keynes (UK). Those are completely different places.
There's also the question of whether that's Japan -> Milton -> UK Distribution
Or Keynes or Japan -> EU -> Milton Keynes -> UK Distribution
As the sole U.K. distributor is possibly the biggest in the world, shipping stuff to one of the smaller eu based distributors first seems unlikely before or after brexit. The shimano shortage seems less bad currently in the U.K. than the rest of the world, particularly the U.S.
And yet the Shimano UK catalogue is produced by Shimano Europe BV
It's not so much who the distributor is as where/who the distributor purchases their physical stuff from, does it come from Shimano in Japan or Shimano in Europe.
Politically it would make sense for Madison to get their stuff direct from Japan now, assuming Shimano will do that, looking at the distributors for non-EU europe there is significant overlaps so wouldn't be surprised if they just get that from EU warehouses with associated extra costs.
As for Madison's size again looking at the distributor list, I'd have thought either Paul Lange with Germany, Czech, Slovakia, Hungary and the Ukraine or Euro Trade with Albania, North Macedonia, Montenegro, Bosnia & Herzegovina, Bulgaria, Croatia, Cyprus, Romania, Serbia, Slovenia, Greece and Kosovo would be bigger than Madison who only have 65m people to sell to without extra barriers (chances of them holding onto Ireland?).
https://bike.shimano.com/en-EU/information/distributors-list.html