St. Just is easier to get to/from the mainline railway, particularly if Skybus started laying on shuttle buses, it is nearer to ISC and if it's the same or similar price as the nightmare of travelling from/to NQY then I'd suggest BIH have, at present, shot themselves in the foot selling the PZE site before having approval(s) for an alternative site.
I'd further suggest that there is plenty of land adjacent to the mainline railway and, so far, BIH went for the 'lazy', 'cheap', option of resiting adjacent to an existing rail station ... I'd suggest that BIH look at a site that requires building a new rail station adjacent to that site.
OK, the Tresco route has no competition but Tresco (alone) ain't going to finance BIH's Cornwall operation!