Although it is possible that it isn't a problem, you haven't actually argued why beyond vaguely being optimistic. Banning older generation lightbulbs will not cause a 20% decrease in electricity over 10 years.
The default position when a country's electricity use drops 20% is to be concerned. If they do the same thing 4 more times they will have returned to the stone age.
If you look at trends, the main driver is the fall in industrial use, which has been consistent since the '70s. This country has effectively abandoned industry with Thatcher, and, unlike others, it has never looked back. British industry is in (un)managed decline, the economy is now entirely driven by services and finance.
Domestic trends are flat or falling softly, driven by those efficiency measures I mentioned. The UK used to have very cheap electricity coming from nuclear reactors that are slowly being decommissioned, which (combined with a typically-botched privatisation of the sector) has driven up domestic prices very significantly, pushing people towards alternatives. For example, there used to be a lot of electric heaters in British homes, which are now invariably replaced with gas-powered ones. That has not meant a significant fall in living standards.
There are many issues with the UK system, particularly a wealth distribution heavily skewed towards one or two metropolitan areas (the centres of services and finance), but falling energy use is not really one of them.
The default position when a country's electricity use drops 20% is to be concerned. If they do the same thing 4 more times they will have returned to the stone age.