At LHR you don't see it as the Speedbirds are parked in "sin bins" awaiting their numbers and wouldn't dare trying it on
Not quite. As I suspect you know they are, along with just about any other operators aircraft, held well short of the runway entry points (e.g. Pluto/Lokki/Loman, until ready) and, gasp, it's not unknown for other operators not to be ready when called, especially those coming out of T1/2/3 for the 27s. I'd agree the BA final figures system isn't perfect, far from it, but personally I've not had to decline a clearance to enter either of the 27's due figures or cabin in several years. OTOH there are issues with compression on the T5 to 09R routings, hence if needed ( and coming out of 5B or 5C it's not always needed) we either to sit on Bravo or those coming out of the southern T5 gates may wait on the terminal apron, leaving the access to 09R via Alpha clear.
The idea that BA flights are routinely held in "sin bins" around the airport waiting for figures is somewhat wide of the mark....