Currently there are 87 airlines serving London Heathrow EGLL. This is going to take some time downloading 
2GO (Cebu Pacific aircraft and liveries)
Aegean Airlines
Aer Lingus
Aeroflot Russian Airlines
Aeroméxico
Air Algérie
Air Astana
Air Baltic
Air Berlin (Incorrect - Ceased operations 2017)
Air Canada
Air China
Air France
Air India Limited
Air Malta
Air Mauritius
Air New Zealand
Air Serbia - Manual download
Alitalia - Manual download
All Nippon Airways (ANA)
American Airlines
Arik Air
Asiana Airlines
Austrian Airlines
Avianca - Aerovías del Continente Americano S.A.
Azerbaijan Airlines
Biman Bangladesh Airlines
British Airways - Manual download
Brussels Airlines
Bulgaria Air
Cathay Pacific
China Airlines
China Eastern Airlines
China Southern Airlines
Croatia Airlines - Manual download
Delta Air Lines
Egyptair
El Al Israel Airlines
Emirates Airline
Ethiopian Airlines
Etihad Airways
EVA Air
Finnair
Garuda Indonesia
Germanwings (Defunct April 2020 - Transferred to Eurowings subsidiary of the Lufthansa Group)
Gulf Air
Iberia Airlines
Icelandair
Iran Air
Japan Airlines
Jet Airways (Ceased operations in April 2019, but is slated to restart operations in early 2022)
Kenya Airways
KLM - Manual download
Korean Air
Kuwait Airways
Libyan Airlines
LOT Polish Airlines - Manual install required (Several)
Lufthansa
MASwings
Middle East Airlines
Olympic Air
Oman Air
Pakistan International Airlines
QantasLink - Eastern Australia Airlines (Incorrect. Domestic regional service, should be Qantas)
Qatar Airways
Royal Air Maroc
Royal Brunei Airlines
Royal Jordanian
SAS Scandinavian Airlines
SATA International (Now known as Azores Airlines aka SATAAirAcores)
Saudia
Singapore Airlines
South African Airways
SriLankan Airlines
Swiss
TAM Brazilian Airlines (aka TAM Linhas Aéreas, now known as LATAM Airlines Brasil)
TAP Portugal
Tarom
Thai Airways International
Tunisair
Turkish Airlines
Turkmenistan Airlines
Unavia Suisse (No information)
United Airlines
Uzbekistan Airways
Vietnam Airlines
Virgin Atlantic Airways
Vueling Airlines
ADDITIONAL MAJOR AIRLINES NOT BASED AT LONDON HEATHROW
EasyJet
Jet2
Ryanair
TUI Airways