it seems to me that an engine failure on such a slow take-off must be more hazardous than when both engines are already on full power
I'd suggest that what you are seeing is aircraft using a de-rated (i.e. not full power) take-off.
Gatwick (like Heathrow) operates a "1,000 feet rule", meaning that aircraft must be at that height (or higher) by the time they are 6.5 km along track, measured from the start of their take-off roll.
In addition, the EGKK SIDs specify a minimum initial climb gradient of 5.5%/5.2% to 200'/600' QFE for 08 and 26 takeoffs, respectively.
Provided a departure meets those requirements (which are closely monitored) then there is no cause for alarm.
Have a read of this:
Gatwick Airport flight performance team report 2016