If a box junction is to be enforced then it should comply with the Traffic Signs Regulations and General Directions 2002 (usually abbreviated to TSRGD). However, if the Local Authority can demonstrate good reason why it is impossible to comply fully with TSRGD, and justify the need for the box junction, then the Secretary of State for Transport can, in exceptional circumstances, specifically authorise the use of a non compliant junction.
If there was a handful of such authorised junctions in the London area then you might believe the above statement, but just look at the list that has been authorised since enforcement began, particularly in 2006. It looks like Transport for London order authorisations by the container load and the Department for Transport (DfT) just rubber stamp them.
It would appear that TSRGD compliant box junctions are becoming the exception rather than the rule. The cynics amongst us might even suspect the DfT is being complicit in allowing Local Authorities to tax farm junctions that present drivers with difficulties.
Judge for yourself; the list is as follows: