We did the same back in April - MAN-LHRT5-LHRT4-JFK
1hr50mins from belts off until arrival in T4 but this included 40mins waiting for bags (BA refused to check through).
Suggest 1h25m is unachievable plane to plane but to give it the best shot I'd certainly be looking at either an airside transfer or a cab if you have to come landside.