If a main contractor is working under a JCT ICD 2011 and he issues a Purchase Order to a Subcontractor without any reference to either the JCT ICD 2011 and or the JCT IC/Sub C is the payment mechanism covered by the JCT ICD 2011, or the JCT IC/Sub/C or the HGCR Act ?
The subcontractor is not likely to be a party to the main contract and as such will have no rights or obligations under it. If the purchase order makes no reference to the terms on which the order is placed, and the correspondence / communications between the contractor and subcontractor which led to agreement of a price do not mention the terms that are to apply, then yes the Scheme for Construction Contracts under HGCR Act will apply provided that the subcontract works fall under the ambit of the Act as construction operations.
If however during the period you were tendering for the subcontract works you have submitted a price subject to certain terms and conditions, and the contractor has replied that he accepts your price but only on his terms and conditions, you have the classic battle of the forms scenario. In which case you may need some help to ascertain which terms apply.