"how exactly are they supposed to advertise speeds now?"
I can't say I have much sympathy for them. But whilst BT have that last mile monopoly there will be no competitive pressure on BT to improve, nor really much pressure from ISPs because for any given house they all face the same limits.
The important thing should be the break up of the Openreach monopoly, and the separation into regional companies that have to report separately to a regulator (and a stronger regulator). This "comparative competition" works pretty well in other regional monopoly situations, such as the ten UK water companies, or the regional electricity distribution companies. But this is clearly beyond the very weak capabilities of OFCOM.