I'm in Australia, so somewhat different ISP situation, but I deal with the same underlying physical layer (VDSL2) that the UK uses for FTTC, and I also had the same goal of getting DSLstats working reliably.

Unfortunately a lot of the documentation you'll find online for accessing modems from behind pfSense makes the assumption that you're using a PPPoE connection for your WAN. My ISP just uses standard DHCP/IPoE and this caused a lot of trouble for me, despite my service being xDSL (since PPPoE is very common for DSL). This made finding the correct information very difficult, it took me hours of searching and reading of posts in fact.

Ultimately, how you access your modem's line stats will depend on how your ISP handles your WAN connection. If it's through PPPoE then Netgate's documentation will do fine and allow you to get DSLstats working pretty quickly. If instead your ISP is like mine (DHCP/IPoE) then I can walk you through setting up the correct NAT rules to save you the trouble.