Guys,I worked in ATC for over 40 years,nearly 30 as a controller in the London Terminal Control Area . There are many reasons why departure intervals may be extended. Not always related to do with how"busy"the airfield may be,it can be to give ATC a break on a particular sector,or relieve pressure on an en route "choke point". Example,BPK,aka Brookmans Park VOR,a major navaid to the SW of Stansted where a number of departure routes from various airfields in the LTMA converge.