When connected via FTTC, the "superfast" epiphet only applies if you actually get speeds of more than 25Mbps.
The actual speed achieved depend on the length of the line, quality of joints, material (aluminium/copper), and gauge (diameter). As "standard", the expectation is that superfast speeds reach out to around 1200m. There are around 6% of lines longer than this.
The targets are indeed to reach 90% of lines which can actually reach the superfast speed threshold. And later, to 95%.
If you don't get a superfast speed from an upgrade, then you technically qualify for further subsidy in future. However, there are other qualification factors that might work against you - such as the one that specifies a " step change" in speed.
As the target rises to 95%, there will be a need for BT to cover people like you - beyond the threshold from an old upgrade - by getting deeper fibre from a secondary FTTC cabinet, or from FTTP. Both kinds of expansion have been seen.