It is not in BA's power to install moving walkways there. Many of us, BA staff have been suggesting this from the time we knew of the walkway, but in the end it's up to BAA, as owners of T5. I think that it would be sensible and then we can allow customers to move freely. It would need to be a series of moving walkways and not one continuous one, as they underground passage is also designed to be used in emergency situations and has security doors that can be closed in various sections.
As for access, you can't press the button on the lifts to get there, unless with staff who have access. The buggy is available to take customers back to T5A, but only if there are 3 or less. If there is a larger group, then they will have to walk, which in reality takes about 5-10mins depending on the individual.
The signage showing the way back to T5A has been increased as people are regularly ignoring the signs in T5A about not going to T5B until the gate is confirmed and therefore need to be directed back to the A gates where their flight is.