There's something odd going on with the #Interrail app with London Blackfriars station as it wouldn't provide journeys from St Pancras, though one stop earlier and it would allow the journey.
#CrossBorderRail
Another odd thing is St Pancras Intl to Portsmouth Harbour suggests a journey via Netherlands and ferry from Hook of Holland to Harwich then back to London and then through to Portsmouth Harbour. Looks like a lack of a link to the local transport at St Pancras.
#CrossBorderRail #Interrail