The BBC has been a BIG supporter of Open Source in general, its just not widely reported.
It makes sense for BT to move aspects of their network away from proprietary vendors who are known to be doing a pretty poor job, causing security and reliability issues.
It always amazes me that almost all routers are built on Open Source but using ancient versions of said software, far from optimal and with potential security vulnerabilities. Its especially annoying that core features (DSL, WiFi) are locked behind binary drivers built against ancient kernels, hampering the end user from switching to better supported third-party firmware.
Even when we can, drivers like Ath10k never perform as well as the proprietary crap in stock firmware. It seems to be that putting optimisations into the binary firmware is a way to avoid feeding back improvements into the underlying Open Source software. They reap the benefits while avoiding their responsibility to improve the software for everyone else, a core purpose of Open Source.