Try the "
So You Want to Learn a Language" site, which has a whole bunch of links (including for e-books) to free language learning resources offered on the web. There's a specific disclaimer that the site only links to public domain/copyright-expired/put up by author themself stuff.
Weirdly enough, it's
Project Gutenberg Australia which hosts
Basic French for Canadian Schools. It's slightly antiquated, but gives a good overall grounding in the basics that's probably a bit more in-depth and structured than most modern courses will give you, and it dates from 1937 with author/editor unknown.
I have no idea what the UK rules are in terms of anonymous publication and the public domain, but it's been over 70 years anyway. And it was a Canadian government publication to begin with. I'm sure the Ministry of Education would have actually wanted it to be used for education, in a non-Orwellian sense of the word.
Hope this helps et bonne chance!