Amazon UK - Free 3-month Kindle Unlimited Trial available until midnight tonight.
Late notice but I only saw this now, and unfortunately I can't share a link since each Amazon member can only use the deal once, then the link generates an error page. But you just need to go to any Kindle Unlimited-eligible book page, and there should be a box detailing the offer below the description.
It says it's not available if you're currently on or trialling KU - not sure about if you took a trial before.
I just signed up, I'm not sure it's worth £7.99/month for me normally, but there are lots of Christmas books available, plus some of the British Library Crime Classics etc so that'll keep me busy until I cancel March 1st!