Hello and welcome,
As you have found out, the Maritime & Coastguard Agency only accept official evidence of sea service.
The 1942 Crew Agreement for RATHLIN official number 164098 is held at Kew in piece
BT 381/2076
That should along with the movement card for RATHLIN should be sufficient to award the medal. The movement card can be downloaded for free here -
BT 389/14/25
Further awards for North Russian Convoy PQ17
WHITEWRIGHT, Edmund Ireland - Chief Engineer - DSC.
Source: Seedies List of Awards to The Merchant Navy
Regards
Hugh