The Amazon store itself no longer offers the free ebooks it used to, presumably as a way of getting people to sign up to Kindle Unlimited or Prime Reading which will give you access to a library of new and old books for a set subscription price. The public domain classics are now offered at a low price, and you can often pick up entire back catalogues or authors for just a pound or two, but look elsewhere and you can still legally download these public domain books in Kindle formats.
One way of getting a huge amount of choice without the restrictions of classics or promotional free books is to utilise your local library. Libraries across the UK are participating with OverDrive's Libby app, which allows you to temporarily download ebooks using your library registration details. You will need to check with your local library to make sure they are participating and what their Libby address is, and you will probably have restrictions on how many you can borrow at a time, but it's a great way to support your local library and get access to hundreds of new books for free. 2b1af7f3a8