I was going through the process online to cancel my Nan's kindle unlimited membership and when going through the process it came up with an offer of £7.99 for three months instead of £23.97 (3x £7.99). She wanted to cancel anyway so didn't do it for her, but thought I would give it a go on mine.
Tried it on mine, went into manage subscriptions etc. went through the clicks to cancel my kindle unlimited and it came up with the same offer for me.
So I am basically getting two months free saving me £15.98
Worth a go if you have Kindle unlimited