From their website
All operators wishing to use the HLS MUST first obtain permission to land from the UK CAA.
This is because landing at the site is:
* In a Congested Area (Rule 5(2)(c) of the Rules of the Air 1996); and
* Against the Air Navigation (Restriction of Flying)(Specified Area) Regulations 2005 as it is within the Specified Area; and
* Against the prohibition from landing off the London Heli-lanes at a private site (Rule 5(3)(c)(11)).