BRL,
In your post you don't say what you would intend to use the pad for. Would it just be for your helicopter, would you allow customers to use it or would you let anyone use it and even charge them?
It is always possible for the lawyers and courts to change their view.
However my understanding of the current position is that so long as the site is within the curtilage of your house (i.e. in the garden) or business premises and that your use of the site as a landing pad is incidental to the primary use, then you do not need planning permission because there is no material change of use. The pad can be used as often as you like within the above limits.
If you convert a piece of land to be a pad that was say an agricultural field, then you are likely to require planning permission. However, if you simply use such a field without structural changes (e.g. putting up a hangar or a fancy opad with lights and bells and whistles) than you can use it for up to 28 days per year without needing PP.
Planning Law and planning permission is about the use of the land: the CAA role is to regulate air safety. The two are quite separate.
In respect of all the above PP stuff, the CAA are irrelevant. They will be concerned that any flight in or out can be done safely and in accordance with the Rules of the Air. In the case of AOC ops, there would be particular requirements that they would require the flight operator to comply with.