Economy 7 is a tariff not a system. How the items are changed between the two tariff's varies.
The old white meter where you have a completely different fuse box for off peak and on peak today is rare, and where it still exists often there are other issues, lack RCD and SPD for example.
There were timers for immersion heaters which did allow you to have a boost during the day. However both line and neutral need to be swapped, and the units tend to cost around £85 if you can still get one.
Even a manual switch which seems a little pointless looking at around £50, so although it can be done, not really practical.
Except in Scotland Economy 7 has nearly gone, and today one would be looking at smart meters. Even when it was common, in the main two immersion heaters would be fitted one on peak one off peak.
Latter installations had single supply and everything could run on off peak at that time, there was a move for washing machines and tumble driers to run over night, then we started to have the fires. October 2014 and February 2018 saw Whirlpool owned by Hotpoint recall thousands of machines.
As to if now safe to use not sure, but would clearly need RCD protection. So to move from fuse box to consumer unit your looking at a lot of money, you need to ask yourself if worth £500 plus to alter the system?
In theory it could be done if you had the correct switch gear to isolate everything not required during peak, (If you do not isolate what is not required, you will end up with a huge overload, causing a fire, burning down the property.) The cost of the switch gear and then paying an electrician to do it safely (So the premises does not burn down) would be cost prohibitive.
Question: If it was that easy why haven't other people done it already? Answer: Too dangerous and too expensive.
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