BT's investment is into the wholesale GEA-FTTC and GEA-FTTP platform operated by BT Openreach. BT Infinity is merely the brand of the BT Retail product operated over this system - it sits on top of wholesale products available to (and used by) other ISPs from AAISP to Zen.
In most cases, line length (and, to a lesser extent, condition) sets a ceiling on achievable ADSL2+ performance - the technology is capable of more but the line limits the available speed.
In a minority of cases, VDSL2 (the same technology used for FTTC) could be used at the exchange to provide faster speeds. However, VDSL2 at the exchange is essentially prohibited by the Analogue Network Frequency Plan, because it would cause compatibility issues with ADSL from the exchange.
Research and development is primarily focused on faster speeds from short lines - improved FTTC and pushing fibre even closer to the end user (Fibre To The Distribution Point and FTTP).
If faster ADSL became commercially available, I think it would be hard for BT Wholesale and the LLU ISPs to justify investing it it, considering that ADSL is typically a narrow margin product that many consumers wish to leave behind for faster fibre-based speeds when available. However, I appreciate this commercial and technological reality is not good news for those falling outside the footprint of fibre-based services.
The best hope of faster speeds from ADSL is bonding multiple lines. This is available now if you are prepared to pay for it - I'm fairly sure it's a standard product from AAISP.