The regulator for adverts is the Advertising Standards Agency, and several years ago they decided that this sort of thing was OK. A key part of this was because "most" people would never get near the limits.

The judgment is at: http://www.asa.org.uk/ASA-action/Adjudications/2008/9/Vodafo...

Of course the number of people affected was a figure provided by Vodafone, and by getting this adjudication in early Vodafone were able to get the precedent set before higher mobile broadband use was common.