Plenty of normal working class people have income from assets, BTL is very common for the self employed to have instead of a traditional pension, many others have accumulated 10s of 1000s in ISAs or had a small inheritance which they've invested, they all work too but chose to save instead of spending it all.
To me, listening to Starmer today, he doesn't intend to tax people who have SFA, i mean how could he? he will have to go after middle income earners.
The very wealthy will use various tax avoidance measures to wrap their earnings & assets away from the tax man, as they always do.