That's a choice. Presumably a cost conscious choice, not a limit imposed by Openreach.
A&A presents each customer with a live updated chart of bandwidth usage, latency, and low-level polling of the DSL modem in the customer's home. If your line drops even if you aren't "using" it for anything, you can opt to get a SMS about that, and choose whether this applies at weekends or overnight.
They even plot the rate of "login"/ "logout" events for the modems, both as a whole and mapped onto exchange locations so that you can see if there's a problem beyond just you got unlucky this time.
Since then the firmware on devices seems to have either dropped those types of features (cost reduction) or restricted them to an ISP-accessible interface ("security"). :(
I worked at a UK ISP a couple years back and lets just say the 'monitoring' that ISPS have is extremely limited. Openreach (the company that manages the phone lines, which ISPs lease) almost certainly has more monitoring but the problem is the customer calls the ISP, and the ISP then tells Openreach - which means there's a fairly huge disconnect between the customer and the engineers fixing the line.