The Openreach FTTC modem is a BT-badged Huawei EchoLife HG612. 
The Huawei HG612 is based on the Broadcom BCM6368/BCM6306 Residential VDSL2/ADSL2+ Gateway System on a Chip (SOC) solution. 
The BCM6368 “combines a VDSL2/ADSL2+ transceiver and AFE with a high-performance multicore MIPS32 CPU, ATM/PTM hardware SAR, hardware packet-processing accelerator, Gigabit Ethernet switch core with four 10/100 Ethernet PHYs and dual GMII interfaces, dual USB Host/Device, multichannel TDM/PCM bus, parallel expansion bus supporting CardBus, and mini-PCI into a single high-performance monolithic device.” 
“The VDSL2/ADSL2+ transceiver delivers 100 Mbps downstream and 50 Mbps upstream (100 Mbps upstream with external AFE), enhanced QoS for IPTV video, dual-latency framing, ATM or PTM physical layer, DSL channel bonding, and embedded operations channels for remote management of the CPE.” 
The firmware contains a MIPS Linux kernel, and a set of user applications similar to those found in another Broadcom-based device called the Neuf Box . On the Neuf Box website there is information on building the cross-compiler toolchain.