If you're referring to the departure speed trial currently ongoing for departures from six LTMA airports/airfields (LGW, LHR, LTN, LCY, STN & NHT) i.e. not just LHR, its purpose is to determine whether or not ATC handling can be improved by not lifting the 250kt speed limit below FL100, and therefore, airspace capacity can be increased.
With Gatwick, Heathrow, City, Luton and Stansted all now handling record numbers of flights it's important to safely squeeze every ounce of capacity out of the existing controlled airspace, given the protracted and painful process involved in extending it.
