You need to get yourself a copy of 'Helicopter Landing Sites - UK'.
If you're on board a pussers grey then your LAC/POAC or Air Ops (in the case of CVS/LPH/LPD) will have a copy of this.
You can obtain a copy from NAIC, if they won't send the book out to you they should help you with any questions if you ask. Failing that, you can ask either Yeovilton Ops or Culdrose Ops and they should be more than willing to assist you.
Off the top of my head I think that there are HLS's at MWS Excellent Upper Lawn, MWS Collingwood Parade Ground and DARA Fleetlands. There are definitely more than that but I can't remember and don't want to feed you false information.