Here's a view from a Yank who visited Europe twice this past summer and whose wife is currently in Blighty:

Don't.

Do you really want to have your products go through customs when they leave the other end of the chunnel? Or do you want to go through customs and immigration whenever you vacation in the Med? Do you think the Northern Irish want the customs hassle when they go to the Republic...and vice-versa? Scotland is stomping to leave the UK -- they would almost certainly go with the EU in that case and again bringing up the customs hassles.

Remember, the EU countries would pass business back and forth without let or hindrance but the UK could not. Since the UK was, in August 2015, a net importer (albeit from both EU and non-EU), are you certain the game would be worth the candle?