"Say what you like about BT, but my rock-solid BT Home Hub router gets updated by BT regularly and automatically."
Which says nothing about its security.
ISPs normally update CPEs (i.e., home routers like yours) only if there's a bug affecting the ISPs network or creating an undue number of support calls. Security updates are generally at the bottom of the priorities list, if it's even on it.
Besides, that TR069 interface BT uses to manage your home hub presents a security challenge of its own and has in the past been part of several widespread hacks of consumer routers.