All terminals have international capabilities (we're just a small island!) - but MIA goes from T3, IIRC, which is not BA's main terminal.

So, if you want to use the public buses, you need to get the the Central Bus Station. This means following signs down a ramp, through a tunnel, along some moving sidewalks (travelators) and up next to the Tube station.

If you want to get the Hoppa bus to the HIX T5, you need to get to T5 first. To do this, you'll need to catch the Heathrow Express train for free - follow the same tunnel and branch off.

Taxis are from a rank outside the terminal. Uber is also possible (current negotiations notwithstanding) although my experience of trying to use them for short hops near airports is far from stellar, but again, YMMV.