Helicopter traffic over central London

[Hoff12]

I live a 0.5 miles north east of King's Cross station, London and notice a lot of helicopter traffic flying south towards  to the centre of London in what appears to be quite a tight "air corridor" roughly over Caledonian Road.
 
Could anyone explain this air corridor and what freqency these helicopters are using? I am not referring to police helicopters (which are easy to identify) but private aircraft.

thanks

[ACW367]

http://www.ead.eurocontrol.int/eadbasic/pamslight-4BD94D77610F3FAAB529DC08576E03AB/7FE5QZZF3FXUS/EN/Charts/AD/AIRAC/EG_AD_2_EGLL_3-2_en_2012-01-12.pdf

[Hoff12]

Thanks. Which frequencies do light aircraft and helicopters flying over central london use?

[ACW367]

They will either be talking to Battersea
http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=173&Itemid=64.html

OR Heathrow/Thames Radar
Heathrow Radar 125.625 ◊ 0700-2030 0600-1930 DOC 60 nm/20,000 ft. ◊ Special VFR and Helicopter flights in the London CTR. These flights will additionally receive a service from the following: Thames
Radar (132.700 MHz): 0630-0700 and 2030- 2200. Heathrow Director (119.725 MHz): 2200-0630. (One hour earlier in summer)
http://www.ead.eurocontrol.int/eadbasic/pamslight-4BD94D77610F3FAAB529DC08576E03AB/7FE5QZZF3FXUS/EN/AIP/AD/EG_AD_2_EGLL_en_2012-05-31.pdf

[DeeJay]

They frequently work Northolt App on 126.45.