Both Gatwick [0] and Stansted are busier than either Washington airport [1], and if you're considering Miami as an alternative to Atlanta then why not similarly ridiculous options like Paris, Amsterdam, Dublin for passengers stuck in Heathrow?
Miami and DC aren't even close to the nearest major airport cities to Atlanta. Charlotte and Orlando are many hours closer and busier [1] in terms of commercial passengers (though still not as convenient as the UK's comparable airports).
Only about a quarter of Heathrow passengers are transiting [2] and a significant portion of those are citizens of the US, EU, UK and other countries who don't need a visa. Maybe 10% of passengers are stuck in limbo, not half of them.
[0] https://en.m.wikipedia.org/wiki/List_of_busiest_airports_in_...
[1] https://en.m.wikipedia.org/wiki/List_of_the_busiest_airports...
[2] https://www.statista.com/statistics/303939/flight-transfers-...