From my experience for charters from battersea with Twin Squirrels, Premiair hard to beat with a Twin Squirrel, although Atlas also very competitive.
Larger helicopters are not always the answer, not that you would find one aircraft for 15 unless you charter a S61 from Ireland which will cost a lot !
As a very rough estimate need to allow at least £400 per head, more if at the weekend due to increased fees at Battersea. This is for a Twin Squirrel.
Would be cheaper if using single squirrels or Longrangers, but not a great deal. Speak to London Helicopter Centre if you need a single squirrel, very helpful.
May find it hard to land 3 aircraft at Guards.
Hope this helps, let me know if you need any more info
Rgds
F09