The full case referenced there is nearly identical to yours. Iberia, with the best of intentions, rebooked the two passengers concerned, but they did make it through to the gate in time for boarding unexpectedly. It was deemed inherent and within the airlines control, and that's all that matters. BA may argue that you could not have made the connection due to HAL's conformance, but that's a bogus argument since there are ways to lift conformance which are under BA's control.
Originally Posted by
cvision
I thought about just buying the EW ticket myself but I already made it upstairs pre-security and that makes you a prisoner of T5; sadly only BA Bronze as of this year. So something for anyone else to consider, if you want to switch terminals to another airline or do a landside transfer, you have to do it before you go through the gates to security. The only way to get out afterwards is (1) if your ticketed BA flight is more than 4 hours out, and (2) at the top of the hour you'd be escorted either down to connections or to immigration, for that you need to report to the main CS desk in T5A.
Or take the transit to C and then back again to Flight Connections.