Keeping GMT all year round, then in summer sunrise would be around 03.45 & sunset around 20.45, most of us don't need sunrise around 03.45 in the summer, & the extra hour of daylight in the evening saves on energy too, so switching to BST between March & October makes sense.
Likewise January/February sunrise would be around 09.30 in Liverpool if we kept BST all year round, so we switch back to GMT October to March.
Their was an experiment done in the late 60s in keeping BST all year round, turned out there was more road accidents in the mornings because of the darkness, which apparently the authorities tried to hush up, but ended up ditching BST in the winter.