Interesting. In my time at BA (pre-T4), the opposite applied - Miami was one of the flights that operated from T1 for several years. I think (though I may be wrong) Chicago was another one.
The services in question were renumbered in the BA290 to BA299 range in order to maintain the principle that all flight numbers below a certain value operated to/from T3 and those above (principally short-haul) used T1.
BA uses the same long/short-haul flight number demarcation (above/below 300) at LHR to this day.