A type of consumer unit sold only with a main switch inside.https://youtu.be/ruXBxOniHvs
This example is very untidy inside, with a mix of old and new colour wiring tangled together.
This type of consumer unit is supplied with a main switch only, additional devices are then selected to fit inside. In this one, most of the protective devices are RCBOS, with one circuit breaker for lighting only.
For most new installations, all circuits will require RCD protection and therefore RCBOs with this type of consumer unit. With this arrangement, any fault will affect a single circuit only.