When did your (land) line rental last include a substantial premium to
offset the multi-hundred pound handset subsidy you received at the start of
the contract?

As long as the UK mobile contract financial model revolves around an upfront
subsidy, it remains "expensive" to continue to pay the full line rental past
the end of the contract - as you are effectively paying for a subsidy you
aren't receiving.  Now the "usual" way (if you don't simply carry on paying
beyond the minimum period) of course is to upgrade to a new handset, receive
a new subsidy and sign to a new minimum contract - but there are at least
now some providers who provide alternatives like a reduced cost tariff (for
the same minutes/texts) instead.  Sometimes threatening to leave can help
these sorts of tariffs to magically appear! :)