see ucrel.lancs.ac.uk/bncfreq/flists.html

Word Frequencies in Written and Spoken English: based on the British National Corpus. Geoffrey Leech, Paul Rayson, Andrew Wilson (2001)

These lists are licensed under a Creative Commons Attribution-Share Alike 2.0 UK: England & Wales License.

You are free:

to copy, distribute, display, and perform the work
to make derivative works
to make commercial use of the work

Under the following conditions:

Attribution — You must give the original author credit.

Share Alike — If you alter, transform, or build upon this work, you may distribute the resulting work only under a licence identical to this one.

With the understanding that:

Waiver — Any of the above conditions can be waived if you get permission from the copyright holder.
Public Domain — Where the work or any of its elements is in the public domain under applicable law, that status is in no way affected by the licence.
Other Rights — In no way are any of the following rights affected by the licence:
    Your fair dealing or fair use rights, or other applicable copyright exceptions and limitations;
    The author's moral rights;
    Rights other persons may have either in the work itself or in how the work is used, such as publicity or privacy rights.
Notice — For any reuse or distribution, you must make clear to others the licence terms of this work.