Just noticed that Amazon gives you free £1 AV Credit to spend on movies and videos on Amazon.
The T&C reads "Enjoy £1.00 credit to spend on movies or TV on Amazon Video when you purchase any Amazon Kindle Book from the Kindle Store (excluding Kindle Unlimited, Periodicals and free Kindle Books) offered by Amazon.co.uk. A maximum of 1 credit per customer applies. UK customers only. Offer ends at 23:59 on Wednesday, September 27, 2017. "
Thanks to raj75 for pointing out that free kindle books are excluded and that 11p book is the cheapest that qualifies. I think this deal is effectively his suggestion.
Thanks to Bad-ger for pointing out a 1p book which is even better.