EGLL DEPARTING B777 CROSSED HOLD LINE FOR RWY WITHOUT ATC CLRNC.

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WE WERE HOLDING IN BLOCK OF RWY 27L. LNDGS WERE ON RWY 27R. TWR ADVISED US TO PLAN TO LINE UP AND WAIT BEHIND THE SECOND A-321 HOLDING S OF RWY 27L. WE THEN TAXIED TO HOLD BLOCK N OF RWY 27L AS DIRECTED. WE OBSERVED AN AIRBUS; B777 AND A SECOND AIRBUS TKOF. I SAID OK WE'RE NEXT; AND STARTED TO TAXI TO LINE UP AND WAIT. XING THE HOLD SHORT LINE; THE TWR SAID WE WERE NOT IN SEQUENCE. WE APOLOGIZED AND OFFERED TO CLR THE RWY AND RE-ENTER THE LINE. TWR SAID THAT WOULD NOT BE NECESSARY; EXPECT TKOF CLRNC SHORTLY. TKOF AND DEP WERE NORMAL. NO MENTION WAS EVER MADE BY TWR ABOUT OUR NUMBER (3; 6; 8) IN THE TKOF SEQUENCE. FOLLOW THE SECOND AIRBUS 321 WAS ALL WE WERE GIVEN. WE APPARENTLY MISTOOK AN AIRBUS 319 OR 320 FOR A 321 AND; THEREFORE; MISCOUNTED THE DEPARTING ACFT. I BELIEVE AIRBUS MODEL NUMBER IS A POOR WAY TO SEQUENCE DEPS IN THE FAILING LIGHT OF DUSK. EXCEPT FOR THEIR LENGTH; NARROW BODY AIRBUSES ARE IDENTICAL. A SIMPLE HOLD SHORT UNTIL YOU ARE NUMBER 2 OR 3; THEN AN ASSIGNMENT FOR SEQUENCE WOULD BE A MUCH SAFER OP.