|snip from John Watson's Blog, link in left sidebar|
This link:Time and Date dot com is to a page listing sunset on January 30th in London at about 4:40 pm. This is the day John says on his blog he went to look at the flat with Sherlock, the night of the serial killer cabbie, the first episode. In the lab, Sherlock told John they would meet "tomorrow evening 7 o'clock." If they had met at 7PM on that date, it would have been dark.
After only a few minutes in the flat, possibly 10 or 15, they are both on thier way out to the crime scene of the 4th "suicide" victim, the pink lady, and the streetlights are coming on.
During the cab ride, while Sherlock gives John his read on him through observing him and his phone and then stating his usual brilliant deductions, night falls in earnest.
In fact, the earliest date they could have arrived in daylight at the flat at 7PM and it become dark about 30 minutes later is March 27th. The day before, March 26th, sunset is at about 6:20 because they are on Standard Time. Daylight Savings Time begins in England on March 27th at 1am. The day before it's far too early. Only between March 27th to possibly April 5th, can they have arrived at the flat in daylight and be traveling in darkness about 30 minutes later.