There was no poor weather at LCY at the time, it was a pleasant eat-in-the-garden evening. The delays were brought about by weather avoidance elsewhere and ATC capacity to handle this, and to accept departures, to the extent that an inbound Dublin-LCY was diverted short into Manchester (on an evening when the Manchester to London trains have all been cancelled due to heat). Most LCY gates are filled in the evening peak, and there's little hard stand space, it is actually quite easy to get to this stage where you can't accept anything else except on a "one out-one in" basis.
Following morning there are lots of pre-planned cancellations, but the ones operating actually seem to be getting straight ins.