If you were at the gate 22 minutes prior to the scheduled departure time then you were involuntarily denied boarding and are entitled to IDB compensation. In these situations it is always best to to take a photograph of the screen as this
should indicate the time and the status of the gate. The photograph should provide corroboration of your version of the timing.
Here is the information from Expert Flyer, unfortunately that doesn't assist you as it shows the flight was late leaving the gate.
Results from https://www.ExpertFlyer.com
Code:
Flight Status Search:
Departing on 13/09/18
Flying BA flight 503
Flight Status Depart Location Depart Date Reliability Arrive Location Arrive Date
LIS Scheduled: 13/09/18 18:45 LHR Scheduled: 13/09/18 21:20
BA 503 Landed Term: 1 Estimated: 13/09/18 18:58 68% / 24m Term: 3 Estimated: 13/09/18 21:28
Gate: Actual: 13/09/18 18:58 Gate: Actual: 13/09/18 21:28
Baggage: 07
Comments:
"DOBA503/13SEP
* OPERATIONAL FLIGHT INFO * BA 503 -1 TH 13SEP18
CITY INFO HOUR (LOCAL)
LIS ESTIMATED TIME OF DEPARTURE 1845
LEFT THE GATE 1858
TOOK OFF 1914
ESTIMATED TIME OF ARRIVAL 2126 LHR
LHR AIRCRAFT LANDED 2123
ARRIVED 2128
*1A PLANNED FLIGHT INFO* BA 503 -1 TH 13SEP18
APT ARR DY DEP DY CLASS/MEAL EQP GRND EFT TTL
LIS 1845 TH JCDRI/M YB/G 32A 2:35
HKMLVNOQSGX/G
LHR 2120 TH 2:35
>"