RCN house style for print and web
underage
one word
under way
the NHS reforms are under way; not underway
Unicef
Unison
University College London
no comma
unmanageable
unmistakable
unnecessary
unparalleled
UK
the United Kingdom of Great Britain and Northern Ireland. Great Britain comprises only England, Scotland and Wales. See Britain
up to date
the nurse’s portfolio was up to date (no hyphens); the up-to-date portfolio
USA
no need to spell out