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To add to this, the domestic flights land into a separate terminal to international departures, so you will need to go through 3 security check points and passports, as well as move between the two inter-connected buildings (which admittedly shouldn't take 10 minutes). Moreover the international airport is very long, and BA often leaves from a gate some distance from central airside security, and can occasionally be a bus departure.
75 minutes is do-able if everything is on time and you don't have a laptop or tablet (you may need to power it on at some or all the checkpoints), but at significant risk since by definition you would be on 2 bookings.