Hi Bookworm,
As you state, at night only IFR and SVFR clearances can be issued in the London City CTR (Class D). Therefore standard separation of 3nm horizontally and/or 1000ft vertically must be applied. However, City Tower can apply reduced separation in the vicinity of the aerodrome when appropriate, and the Metropolitan Police and Helimed helicopters can separate themselves visually, provided that certain criteria are met.