Apart from what is already explained on the topic linked by Montgomery Burns, it must be said that De La Rue is one of the oldest and most respected banknote printers in the world, and has been the primary competitor of the German printer Giesecke & Devrient. They haven't been elected to print any Europa series banknotes so far but their printing facilities keep
assigned letters (H and J) for the printer code if they happen to be printing

or

notes (to be issued in 2019). Probably not happening now, due to the Brexit…
The UK is not part of the Euro Area (= Eurozone) but it has been part of the EU and printing works were carefully chosen within all the EU. Other printers that
have had assigned letters but ended up not having been assigned any banknote production were the Bank of England (A), Tumba Bruk of Sweden (C) and Danmarks Nationalbank (S). So these letters have never been used. And, in fact, they won't be used ever (check Antti Heinonen's
The first euros [available in
pdf], p. 206).
Also, let me tell you that, depending on the last two digits on the printer code (= shortcode), that note could have a market value between 40 € and 80 € at the moment. It could go up to 600€+ if it was a H004A1. I am not a collector though, just someone who follows the collector forums out of curiosity.