It makes sense these days to replace that with lower power, lower voltage wiring

I don't think it does. As you say, double the voltage, halve the current - so for going down from 240V to 12V you multiply the current by 20.

Right now, in my house, the ring mains is fused at 5A per floor

In the UK, domestic lighting is wired with 1.0mm or 1.5mm cable, which is safe for carrying at least 12A so there would be no problem increasing the fuse to do this. But using it to carry 10A over, say, 30 metres around a house lighting circuit is going to result in a voltage drop of about 13V in the cable alone, plus any additional drop in switches and junctions.

Because lighting circuits aren't wired in rings as standard, what you'd end up with is a completely different voltage at the start of the lighting circuit run compared to the end, massively significant for 12V or 24V LEDs. You'd have to fit much larger cable and that would offset most if not all of the savings in using low voltage lights; the massive loss of power in the wiring would offset anything left.

The other practical considerations are that it would be unrealistic to have low voltage DC for most household appliances, so you'd effectively have two separate power systems, and they'd need to be safely isolated from each other. And, of course, you'd need something to generate your low voltage, high current DC in the first place. Hard to see any advantages anywhere.

It makes sense these days to replace that with lower power, lower voltage wiring

I don't think it does. As you say, double the voltage, halve the current - so for going down from 240V to 12V you multiply the current by 20.

Right now, in my house, the ring mains is fused at 5A per floor

In the UK, domestic lighting is wired with 1.0mm or 1.5mm cable, which is safe for carrying at least 12A so there would be no problem increasing the fuse to do this. But using it to carry 10A over, say, 30 metres around a house lighting circuit is going to result in a voltage drop of about 13V in the cable alone, plus any additional drop in switches and junctions.

Because lighting circuits aren't wired in rings as standard, what you'd end up with is a completely different voltage at the start of the lighting circuit run compared to the end, massively significant for 12V or 24V LEDs. You'd have to fit much larger cable and that would offset most if not all of the savings in using low voltage lights; the massive loss of power in the wiring would offset anything left.

The other practical considerations are that it would be unrealistic to have low voltage DC for most household appliances, so you'd effectively have two separate power systems, and they'd need to be safely isolated from each other. And, of course, you'd need something to generate your low voltage, high current DC in the first place.

Regarding standards - in the UK, BS7671 recognises low voltage circuits as SELV:

Extra-Low-Voltage system (i.e. normally not exceeding 50 V a.c. or 120 V ripple-free d.c.) which is electrically separated from earth and from other systems in such a way that a single fault cannot give rise to an electric shock.

I'm not aware of any standard for repurposing mains wiring for SELV, and the earthing arrangements would need to be changed to ensure electrical separation.

It makes sense these days to replace that with lower power, lower voltage wiring

I don't think it does. As you say, double the voltage, halve the current - so for going down from 240V to 12V you multiply the current by 20.

Right now, in my house, the ring mains is fused at 5A per floor

In the UK, domestic lighting is wired with 1.0mm or 1.5mm cable, which is safe for carrying at least 12A so there would be no problem increasing the fuse to do this. But using it to carry 10A over, say, 30 metres around a house lighting circuit is going to result in a voltage drop of about 13V in the cable alone, plus any additional drop in switches and junctions.

Because lighting circuits aren't wired in rings as standard, what you'd end up with is a completely different voltage at the start of the lighting circuit run compared to the end, massively significant for 12V or 24V LEDs. You'd have to fit much larger cable and that would offset most if not all of the savings in using low voltage lights; the massive loss of power in the wiring would offset anything left.

It makes sense these days to replace that with lower power, lower voltage wiring

I don't think it does. As you say, double the voltage, halve the current - so for going down from 240V to 12V you multiply the current by 20.

Right now, in my house, the ring mains is fused at 5A per floor

In the UK, domestic lighting is wired with 1.0mm or 1.5mm cable, which is safe for carrying at least 12A so there would be no problem increasing the fuse to do this. But using it to carry 10A over, say, 30 metres around a house lighting circuit is going to result in a voltage drop of about 13V in the cable alone, plus any additional drop in switches and junctions.

Because lighting circuits aren't wired in rings as standard, what you'd end up with is a completely different voltage at the start of the lighting circuit run compared to the end, massively significant for 12V or 24V LEDs. You'd have to fit much larger cable and that would offset most if not all of the savings in using low voltage lights; the massive loss of power in the wiring would offset anything left.

The other practical considerations are that it would be unrealistic to have low voltage DC for most household appliances, so you'd effectively have two separate power systems, and they'd need to be safely isolated from each other. And, of course, you'd need something to generate your low voltage, high current DC in the first place. Hard to see any advantages anywhere.