UK
England, Wales, Scotland and Northern Ireland; no need to write “United Kingdom” in full: write Britain or (the) UK.
Ukraine
Not the Ukraine.
uncharted, unchartered
If you embark on an activity that people haven’t experienced before, you’re entering uncharted waters or uncharted territory. The term is often incorrectly written as unchartered, which would mean “not having a charter or written constitution”.
until, till
See till, until.
USA, US
Either is correct. But avoid using the two different forms close together.
user ID
Two words, ID in capitals; plural: user IDs.
username
One word.
utmost, upmost
When you do your very best to achieve something, you’re doing your utmost. And a thing can be of the utmost importance. Sometimes it’s mistakenly written as upmost, which means roughly the same as uppermost, which in turn means “on top” or “highest” (in a physical sense).