How are short haul arrivals parking slots for T5 allocated?. On flights from MUC sometimes we get parked out on the ramp and catch a bus to the other times go straight to the terminal.
Its an apron, not a ramp... unless you're American.
(although in your context, its actually "remote stands" vs "contact stands")
I would suggest its predominantly a factor of how soon the aircraft is scheduled to be used again.
If you're arriving on something that's due for a swift turnaround then they'll find a spot for you at the terminal. But if you're coming in on a late flight and the aircraft isn't going to be used until the next day, then they'll stick you on a remote stand.
There will also be other factors such as whether you've got any disabled passengers to deal with....
Maybe this document might give you some insight, not LHR, but from DUB, its their "Stand allocation rules" document....
here