Are you checking bags?

Will you be arriving in UA F or C class and eligible for fastrack arrival (non EU passport)?

Are you departing on BA or AF in C class?

Do you have a passport that does not require a UK visa?

Answer these questions and you will probably get better info about whether to take BA or AF and whether to use the FCC (flight connections centre) or to clear UK customs and either walk to T2 or take the free HEX transfers to T4.