I think it would still get posted about around the net that OcUK are doing a huge giveaway competition and would still get people looking at the shop website, even if it were limited to loyal forum members. Imo anyway.
Not nearly to the extent that it does when people can actually enter.
I've never seen a post on another forum saying 'Hey guys, check out this cool competition on another site, chances are none of you can enter because you haven't been registered for 3 months but isn't it nice?'
No one would care what sort of giveaway they were doing if they couldn't take part in it (unless it was a seriously mental, like £10,000+ sort of giveaway maybe).