Worked for the Economy 7 tariff... was cheaper to use the washer/drier in the evenings.
E7 was always a solution for the (state owned) energy industry's problems, not yours. The very nature of any time of use tariff means that if you get cheaper than standard off peak power, you have to have more expensive peak power (otherwise somebody's taking a big loss, or you've invented some form of perpetual motion machine).
The consequence of higher peak and lower off-peak rates is that there is a magic proportion of power you have to use off peak to be better off. And depending on the tariffs, that's somewhere around 35-45% of your total demand, which is quite a lot to use between 12:00 and 07:00.
When I worked for an energy supplier, we reckoned that at least one third (possibly more) E7 customers were paying more than they would be on a flat rate tariff. As a rough guide, you have to have time controlled electric storage heaters set around the E7 period as a minimum. North of Leeds that might be enough on its own, south of Leeds you probably need to run all your dishwashers, tumble dryers and washing machines in the E7 period as well. Obviously, if you'rte on night shifts, or have some non-standard use pattern, things will be different.