Finding legal downloads for UK-based fantasy ebooks isn't as tricky as some make it out to be, but it depends heavily on what you mean by 'fantastic fiction'—are we talking mainstream publisher fantasy or indie stuff? The big names like Tor UK or Gollancz usually sell through all the major retailers: Amazon's Kindle store, Kobo, Google Play Books, and Apple Books. Most of these let you download an EPUB or their proprietary format after purchase. For a wider selection across stores, I'd check out eBooks.com; they have a decent UK fantasy section and sell DRM-free EPUBs from a bunch of publishers, which is pretty neat because you can actually move the file around.
A lot of UK authors are also going direct these days. I've bought EPUB and MOBI files straight from the websites of authors like Adrian Tchaikovsky or Fonda Lee. It's worth poking around an author's official site—look for a 'buy direct' or 'shop' link. Subscription services are another angle. Scribd has a ton of UK fantasy, and while it's a streaming model, they do have an offline reading feature in their app. It's not a download in the traditional 'own the file' sense, but it works for reading on the go without cellular data. Honestly, my main advice is to avoid the sketchy 'free ebook' sites; half the time the formatting is garbage, and you're never sure if the author sees a penny.
I've had good luck with Kobo Plus as well for discovering newer UK fantasy series. You can download for offline reading through their app, and they often have titles that aren't on the bigger subscription services. The selection feels a bit more curated than the overwhelming Amazon Kindle Unlimited catalog.