Actually, you can plant as soon as the last hard frost is past. As long as the sun is gaining each day, and the length is over 12 hrs, and the plant gets at least 5 hours of direct sunlight, it will veg.
Think of it this way.... in the wild, weed disperses its seeds upon death in the late fall. The following spring, it pops....it doesn't wait for 14 hours of daylight. It pops when enough moisture and warmth heats the ground, and that's it.
Here is a useful link. You can make a yearly, or monthly calendar according to day length here...
www.sunrisesunset.com
Here is the link for March 2010 London.
http://www.sunrisesunset.com/calendar.asp?comb_city_info=London,
You may plant ur seeds around March 12th (they take a week), and you tilt past 12 hrs on March 19th.