You can buy a generic service book from ebay for a couple of pounds.
For my MiniI I intend to just keep all receipts and fill in the service book. Usually by the time I sell my cars it makes no difference to the price I get as long as I have a full service history and every garage I have sold to in the last 40 years has accepted my signature against each service. I have always carried out my own services/repairs once my cars are out of warranty and will continue to do so until I run out of 10mm spanners.
I may programme my own service info on the display at sometime in the future as it looks easy but I will still keep my hard copy service book up to date.
It's personal choice and your capability whether you use Mini main dealer, Indy or home.