the exact contents of the pack will not be decided until the end of march/ early april. the outline is that it will contain the local search, deeds ( maybe ), a survey, an energy efficiency assesment of the property and land registry plans.
it will be written or handled by solicitors or specific HIP companys, maybe even the estate agent. the estate agent for example could offer the pack for free and the charge for it would be absorbed in the overall selling fee. if you want to change estate agent if your house does not sell then you will probably have to buy the pack off the estate agents. all of the above is rumour and assumption. We should all know the official requirements by the end of this month.
The important bit is that you will not be allowed to put your property on the market until the HIP is completed. it could well be 4 weeks + for the HIP to be prepared.