I agree the frame itself looks older than 1970s, based on the lamp mount on the front fork. Plus the rear triangle/drop outs look like this 1947 Claud Butler Avant Coureur: Claud Butler Avant Coureur - 1947

Apparently the "real" Claud Butler frames were made up until 1956-57, according to this article: Claud Butler - King of Lightweights . I'd like to think you have a "real" one but don't know enough about them to say for sure. Reach out to "Oldpeddaller" on this forum, or maybe he has already seen this, he knows alot about UK bikes of this era!