The fog was reported in English airports this morning, which may have caused disruptions. Additionally, Paris Orly reported capacity issues.
London City, Heathrow, and Stansted were affected in England, and Brussels, too.
— EUROCONTROL (@eurocontrol) April 8, 2019
As always, air passengers affected by this delay have all the usual rights granted to them under Regulation (EC) 261/2004. For delays lasting more than two hours, the airline has to provide you with free meals and drinks. You should also be allowed to make two free calls, use email or send a free fax. If you have to wait for the flight for one or more nights, the carrier has to provide you with free accommodation at a hotel, including transport to/from the airport. In case they’re unable/unwilling to do this, save the receipts: all “rational” expenses (no VIP penthouses, caviar or limos) should be refunded afterward.
However, low visibility/weather conditions and issues with the operation of the airport are considered to be exceptional circumstances as defined by (EC) 261/2004 – which means you can’t claim disrupted flight compensation! Other rules still apply, so the airlines should still take care of you.