While I’m sure you’re correct about the commercial reasons behind the biometric system, I’m not sure the system you describe is better. As a frequent domestic-to-international (and vice versa) passenger, the transit process is pretty slick and certain better than having to change terminals.

As I understand it, when designing T5 (and T2) there were only limited options.

Either build separate and fully segregated domestic facilities - either as a satellite terminal, or by taking the northern third or so of T5A and completely partioning it. I have my doubts that the facilities and lounges in a domestic only terminal would be as good.

Or require all international transiting pax to clear the UK border as per the US setup. Which would make Heathrow a less attractive international transit point.

Biometrics were the compromise to allow the mixing of domestic originating and those that had cleared the UK border heading to a domestic destination together with international transit pax who had not cleared UK immigration. Setting aside a the commercial benefits to HAL, as a passenger I think the setup brings plenty of benefits and creates a decent transit experience, despite the very rare chance of a hiccup with the technology. Though BA/HAL should definitely look to how that could be minimise and come up with a better mitigation strategy if/when it does fail.