I suspect rents and wages at SYD are significantly cheaper than those in T5 at LHR.

Would the demand be there to make a McDonald's more viable than what is in its potential spot at T5? I suspect not, or there would be a McDonald's there.

No idea how much it would cost to run a McDonald's in T5 but they'd need to sell 45 meals an hour (assuming a 12 hour day) at £5/ meal to take in £1,000,000 a year, which I suspect would not cover much once, you take profit margin, rent, wages etc into account