A self build house is one where the owner actually gets involved with the physical build themselves or directly manages the building work. With Custom Build the owners oversee the design and the build process but they are not physically involved in the building work itself. A contractor is usually employed to manage the build and on multi house sites might be building several houses for different clients at the same time.
In reality there is not a clear cut distinction between the two and people build their houses in all sorts of different ways. Some people opt for an entirely turnkey solution where they can be in a different country while the build is going on while others will be involved in the build on a day to day basis. Talk to a House Manufacturer or put together your own team and build your own way.
The UK government is actively encouraging councils to support Custom Built Homes. All councils are now required by law to keep a list of people who want to build their own home. Councils are then required to ensure that enough serviced building plots are granted permission to meet this demand. For more information and links to council registers visit the Right To Build Portal
"Self-build isn't just for retirees or people who want to build an amazing house out of goat hair. The reality is that custom building a house can be a cheaper way to provide a home for you and your family." - Read more in this Guardian article.
Info graphics from A Right To Build by Architecture00:/ and Sheffield University