As a practice when parties are aware of it then no problem with it.

Problem is many people get so confused when dealing over the phone with these things, or don't read T&C's and if these stretch to a few pages do you blame them?

Also people may simply forget, and the reminder letter - given the level of junk mail, and that they know their bill is direct debit, many may ignore the post warning.

Comparisons to car insurance are different due to the legal requirements and costs of lapsing, so people generally pay more attention, but not always, e.g. recent issue where some did not get Certifcate of Insurance from Tesco and never noticed, i.e. policy was not actually running.