roadside emmission tests have been happening in london for quite a while, long before the february rule changes, both cars and goods vehicles,
as for the DPF only need to be working at colder engine temperatures, sorry, thats wrong, particulate matter is released by diesel burners throughout the temperature range, the reason they block quicker on engines that do short journeys is because the DPF doesnt get hot enough to burn off the residue...but the fact is, when engine is cold, they still prevent the PM from being released to atmosphere.
the catalyst is also required to work throughout the range, but its important that the system comes into full swing as quickly as possible..hence the need for heated oxygen sensors, to speed up the process where the catalytic can cope with emissions.
in both cases, you will get an accurate readout on emissions with a hot engine, not a cold one
people offering DPF deletes and promising no problems with passing mot's when done, are living on borrowed time...if anyone goes ahead anyway, then strongly suggest they ask them to back up those promises in writing, in other words, let them put their money where their mouth is.