Fully lemmatised wordlist based the BNC (World Edition) is available for download (zipped 4.24 M, uncompressed over 16 M). The file is in plain text delimited by tabs, but it is too large open using Notepad. Would suggest opening it using Excel or Word.
The lemmata is used with WordSmith 3 and 4 to lemmatize wordlists, i.e. to join etymologically related words together. First produce or open a wordlist using WS Wordlist, and then click on the icon for lemma-match and load the lemmeta in WS3, or select "Compute - Lemma match" from the menu and load the lemmeta in WS4, to produce a lemmatized wordlist.