Looks cool, but do you have to own a London Power amp?
THD Hotplate and Weber Mass will probably always be the industry standard.
Surprisingly, you can get similar results with a cheap Alesis Nanoverb - $60 used.
I know it's not the same concept as attenuation or "power scaling", but if your amp has an FX loop, you can definitely use it to bring a loud amp down to practice amp levels. If you have a DSL or Single Rec halfstack in your home, you owe it to yourself to at least try a Nanoverb. You simply dial up the same setting you would in a loud situation, then use the Nanoverb's output knob to pull the overall volume down.
At the same time, you can mix in an appropriate amount of reverb so the sound isn't so dry and "in your face." Of course you aren't smacking your power tubes with as much preamp volume, but if your amp is turned up fairly high, the preamp signal is hitting some hot tubes, so the effect is similar. You won't be damaging either your amp or the Alesis Nanoverb. I use this on my 50W Jubilee stack at home. It sounds like 'stadium' tone at a level that won't bring the cops to my door.