Thursday is All Saints' Day and a Holy Day of Obligation - which means we're meant to go to Mass. There are services at Our Lady at 8am, 10am and 7.30pm.
Friday is All Souls' and, while not a Day of Obligation, many people want to pray for their dead. There are Masses at 8am, 10am and 7.30pm.
The churches in central London all say Masses through the day, with services particularly concentrated around lunch time and in the early evening, so finding a service you can attend should not really be a problem.