Kerosene Kraut,
Here is the report about an Evergreen 747F diverting into EGLL due IFSD. Flew right over crowded bits of London more than once, despite being edgy about the likelihood of the remaining three donks staying lit.
Whilst the crew had charts for EGSS, they elected to go for EGLL as that was the one they had already seen and could keep track of in CAVOK conditions. They wanted to get down PDQ as the thought they were losing the lot.
They asked for vectors to EGLL and that's what they got:
the stated requirement of the aircraft commander to land at London Heathrow was facilitated.
So if a commander asked for vectors to EGLL due medical emergency, one suspects they'd be given just that. And it would not, I suspect, matter whether the operator was LH, AF, BA, SQ, EV or anyone else.