Maybe you once added "UK" manually and JOSM remebers this?

No problem. I fiddled around with my maproulette-challenge for finding wrong ISO-country-codes (https://maproulette.org/browse/challenges/52083) and than I saw about 600 hits for this area. Thank you for adding the data in the first place. Adding 600+ addresses is a lot of work, correcting two letters in a mass-edit is only a matter of minutes.