I just did the math on a trip early April to Miami. Using Google Flights I came to LHR-MIA-LHR in J at € 3940 (BA A380 hardware). The same long haul but ex-Europe, in this case Amsterdam came out at:
AMS-LHR-MIA-LHR-AMS in J at € 2590 (All on BA). An extra ticket LHR-AMS-LHR in Y would add € 117.
The financial benefit would be € 1239 or 31%.
Couldn’t get the site to use the UKP on my PC, but anyway, the advantage is rather clear. Other airports may even be cheaper.