The cycles of marijuana are determined by the amt of light per 24 it recieves.
Outside, in the late summer (approx. mid-Aug.), increasing dark periods per 24 trigger the flowering process. From then it takes approx. 2 months to reach peak ripeness.
There are 2 ways to get an earlier harvest.
Onme is to raise the plants indoors under 24/0 HID's, then put outside in the spring. The plant will react to the cut in overall light lengths and begin to flower. And get buds by July 01. But it doesn't always work.
The other way is to plant the plants inside a frame of some sort. Then you can cover the plants up in the afternoon, 12 hours later remove it, 12 hours later replace it, and so on and so on.
For 2 months. Twice every 24 hours you have to be there, same time each time.
If you miss a day you will either re-set the "clock" and have to start all over again, if the time varies you run the risk of turning them all into herm's or males.
Generally, the UK does not have good growing conditions for growing mj, which prefers long hot dry summers.