Oxford English Dictionary

The Oxford English Dictionary (OED) was first published in Oxford, England.

OED stands Oxford English Dictionary. It's category would be REF (reference book).
Def

The Oxford English Dictionary (OED) is considered the most extensive English dictionary, containing over 600,000 words and tracing their origins and historical usage back over a thousand years. It is a comprehensive resource for understanding the evolution of the English language.

The answer to the crossword clue "UK lexicon" is "OED" which stands for the Oxford English Dictionary.