From the end of March till the end of October every year, the London session opens by 8 am in the morning. But from November till middle of March the London session opens by 7 am Nigerian time.
The reason for the discrepancy in London session opening time in Nigeria is due to something called daylight savings.
During daylight savings the London government order that their clocks, be adjusted forward by one hour; making London time to be the same as Nigerian time.
But when daylight savings is over, the London government orders the clocks to be reversed back by one hour, and so Nigeria will then be ahead of London by 1-hour. The implication is that the London forex session that used to start by 8 am in Nigeria during daylight savings, will then start by 7 am after daylight savings ends and London clocks are reversed.
It's a bit confusing to grasp but I hope this helps.