A friend of mine just received a surprising £5 promotional code from Amazon, claiming that he took part in a Prime Video campaign. I dug a bit and found out more about the offer which you may be eligible for.
You can view the offer page here. Like always you should see your eligibility at the top of the page. Note that you must a Prime member to be able to participate, and in theory you should not have streamed any Prime video in the last six months.
If eligible, you just need to stream any Prime video by May 3, and a £5 promotional code should arrive in your mailbox within a couple of weeks. Your basket value needs to be at least £20 to redeem the code.