A day after I migrated my line, BTW updated their records for the new CLI. I thought that the B range was very close to the A range and actually assumed it was just because it was a new CLI. But nope, the VDSL estimates across both CLIs and for the address have been updated and are consistent.
I was actually unaware there had been any changes until I read this and then checked everything!
Anyway, it seems they may have raised the downstream handback slightly for range A (perhaps unrelated?), and range B is literally 0.1 Mbps less for the "low" downstream compared to A. The downstream handback has been increased by just over 10 Mbps(!) on range B.
Still no sign of G.fast though.