Does an actual dimmer switch exist?
You're making it harder than it is. You're going to laugh when you realize.
It's common to have fixtures that take multiple bulbs
In the fixture wiring, they typically give separate access to each light's &quot;live&quot; wire.
So simplicity itself. Run power to the switch, then run 3-core wire from switch to light.  Now you can use the standard UK connection block to wire it so each bulb is controlled by a different switch.
Now, put a tiny bulb in one socket and a big bulb in the other.