If you're paying for the highest speed BT offers at your location and you're near the limit for that speed, they don't have any capability to overprovision a few extra mbps on that circuit for guest Wi-Fi use. The closest they can do is provision a new circuit entirely, but that isn't a very cost-effective solution just to provide Wi-Fi as it requires another physical pair, another DSLAM port, and more expensive CPE that can support 2 pairs.
Comcast on the other hand is a cable provider and due to the shared architecture of DOCSIS, can conceivably do that. In fact, since they're saying guest Wi-Fi users will not impact your connection speed, I am betting that is exactly what they are doing - guest Wi-Fi users probably have access to any unused capacity on the node.
* maybe they have FTTP in some areas (like pretty much every telco) in which case this won't apply. And I know they have FTTC in many areas too, but that's still xDSL and these shortcomings still apply (just to a lesser extent since the loops are much shorter).