The 30 miles is an absolute maximum on a full charge.
You are correct in that you are left with a 148 BHP petrol engine once all charge is depleted.
In between times, the electric motor assists the engine to make the overall efficiency better. Regenerative braking is used which can replace some charge into the battery.
Plug-in hybrids are especially good for people who do frequent short journeys, but to get the best from them, they must be charged up regularly.
Long journeys with a depleted battery is pointless in a PHEV, as you are left with a relatively thirsty petrol engine
Witness the long term What Car? Volvo XC90 T8 PHEV which averaged 30.5 MPG!