To add to the above, if charging using Economy 7, or Economy 10, don't change anything, as the off-peak times for both of these tariffs are always set to GMT. For example, our E7 off-peak period is from ~00:00 to ~07:00 GMT, all year around.
It catches one of the time switches I have out every year, the one that controls our hot water boost. That time switch is a "smart" one, that automatically adjusts for the BST/GMT change. I then have to manually re-programme it every time it does this, because we're on E7. One day I'll get around to replacing it with a dumb time switch that doesn't cause this hassle twice a year.