EU compo ''kick in'' is calculated on the delay of the arrival time not on departure time - arrival delay has to be at least 3 hours or more late on arrival to apply for compo ...compo then on a rising scale dependant on route distance and time delayed
However the drone delays and/or any delays related to the knock on effects will not see any compo due to being out of LGW's control hence exceptional circumstances
You are entitled to any out of pocket expenses incurred however (keep receipts) and the airline should have after a 3 hours provided pax with sustenance and/or accommodation if delayed overnight
Although LTN and other airports in the UK suffered knock on effect delays due to the LGW debacle you will not get any EU compo address.
Even if flights were delayed due to no crews or aircraft say at LTN (an operational reason) due to them being all over the place i doubt whether you would be able to seek redress as the airline would argue the LGW circumstances was the cause.