The canonical edition of all three of Douglass's autobiographies (we're reading the last and least read!) is the Library of America edition. It can be purchased at a discount here:
https://www.amazon.co.uk/Autobiographies-Narrative-Frederick-Douglass-American/dp/0940450798
There used to be a paperback, which can be purchased from Amazon.com and it "might" be able to be shipped to the UK
https://www.amazon.com/Douglass-Autobiographies-Library-America-Editions/dp/1883011302/
An epub version is attached.