Small point. The devices called out by the authors are not routers, they are MODEMS which are only installed on Fibre To The Cabinet installations. (These modems are then connected, usually, to an ISP-supplied router such as the BT Home Hub or whatever equivalent your ISP chooses to use.)

Fibre installs are by no means the majority or even the standard in the UK. But obviously the general "don't trust your ISP router if you really think the security services are after you" advice would seem to hold good no matter how you're connected.