The ATM can be used to verify mode C at Gatwick also - by the same MATS1 rules.
The time it's most often used at Gatwick is for seperation between SAM and BOGNA departures from 26. These SIDs diverge at approx 6nm and therefore we cannot seperate them by means of a standard 60 second departure split, or indeed visually, since the divergence is too distant. Instead we can restrict the second aircraft to 3A and maintain visual seperation until the leading aircraft reports out of 3A or a verified mode C indicates as such.
As vespasia says, you may not be on these SIDs, since longitudinal separation can erode in many other instances - but this is the most common. Also the part about requiring coordination is correct, especially since the aircraft are usually transferring to discrete frequencies.
and why every time?
This is definitely not the case at Gatwick - although you may notice it more during the summer months for two reasons. The density of traffic is such that we're more likely to use this technique to maximise the capacity of the runway and the number of aircraft flying BOGNA/SAM routes often increases relative to the others due to increased numbers of flights to France, Spain and Portugal.
You will need to pass your passing (!) altitude everytime on first contact with a radar unit - so that the mode C can be verified.