I have to admit that in the 5 or so years I have been writing news I have never seen a UK carrier advise customers NOT to download a system update.
The warning to customers advising them not to download iOS 6.1 comes from UK carrier Vodafone who said the following to Pocket-lint: "We’ve just told iPhone 4S customers not to download iOS 6.1 while Apple works on a fix."
The fix mentioned is causes 3G issues so you can understand why they have issued the stay away warning to their customers.
Are you affected by this? Have you already downloaded it?