A problem in the UK is that while advertising guidelines exist and a body such as Clearcast exists to perform ad clearance there is still the ASA that will react to complaints, and as with any guideline there is always people who will have different interpretations ensuring a steady flow of complaints.
Virgin Media objected to three adverts for the BT Infinity 1 service when it re-launched as an up to 52 Mbps service instead of its old up to 38 Mbps product (upload speeds stayed the same), and Virgin Media said the adverts were misleading because they 'implied that BT’s up to 52 Mb service was the fastest maximum speed for a lowest-priced tier available in the UK.
The complaint was upheld by the ASA, but the ASA did not appear to say that this was because Virgin Media and its entry level service was cheaper, but rather that the BT adverts had been vague in how it defined who it was comparing itself with and thus people may have understood the adverts to be referring to the whole UK market.
Essentially because BT did not make it clear they meant the top 4 or 5 major providers the adverts have been banned and BT must 'ensure that future ads made clear the basis of the comparison “fastest fibre speeds as standard”'.
The speed battle between Infinity 1 and the 50 Mbps Virgin Media service is pretty tight one, BT Infinity has a maximum connection speed of 55 Mbps, thus max speed in real world is around 52 Mbps, and Virgin Media goes one step further by generally over provisioning the connection speed of its products, so that while its sold as up to 50 Mbps if a local network segment is performing well speeds in excess of 50 Mbps are possible. Virgin Media is usually very careful in its adverts to include the 'fastest widely available' tag line when talking about speed, because of the fact that 1.8% of UK premises that can match or exceed the fastest VIVID service currently and Virgin also link to their SamKnows testing (which tests a small number of connections lots to determine variations in speed over time) to show what these show for services (e.g. up to 50 Mbps has an average 46.89 Mbps at peak times compared to a 24 hour average of 52.28 Mbps).